Standard solid-state methods produced black crystals of the compounds BaCu0.43(3)Te2 and BaAg0.77(1)Te2 at 1173 K; the crystal structures of each were established using single-crystal X-ray diffraction data. Both crystal structures are modulated. The compound BaCu0.43(3)Te2 crystallizes in the monoclinic superspace group P2(alpha beta 1/2)0, having cell dimensions of a = 4.6406(5) angstrom, b = 4.6596(5) angstrom, c = 10.362(1) angstrom, beta = 90.000(9)degrees, and Z = 2 and an incommensurate vector of q = 0.3499(6)b* + 0.5c*. The compound BaAg0.77(1)Te2 crystallizes in the orthorhombic P2(1)2(1)2(a00)000 superspace group with cell dimensions of a = 4.6734(1) angstrom, b = 4.6468(1) angstrom, c = 11.1376(3) angstrom, and Z = 2 and an incommensurate vector of q = 0.364(2)a*. The asymmetric unit of the BaCu0.43(3)Te2 structure comprises eight crystallographically independent sites; that for BaAg0.77(1)Te2 comprises four. In these two structures, each of the M (M = Cu, Ag) atoms is connected to four Te atoms to make two-dimensional layers of [MxTe4/4](n-) that are separated by layers of Ba atoms and square nets of Te. A Raman spectroscopic study at 298(2) K on a pelletized polycrystalline sample of BaAg0.8Te2 shows the presence of Ag-Te (83, 116, and 139 cm(-1)) and BaTe vibrations (667 and 732 cm(-1)). A UV-vis-NIR spectroscopic study on a powdered sample of BaAg0.8Te2 shows the semiconducting nature of the compound with a direct band gap of 1.0(2) eV, consistent with its black color. DFT calculations give a pseudo bandgap with a weak value of the DOS at the Fermi level.
Title:
Natural versus synthetic quasicrystals: analogies and differences in the optical behavior of icosahedral and decagonal quasicrystals
Authors:
Bindi, L, Stanley, CJ
Source:
REND LINCEI-SCI FIS 31 (1):9 10.1007/s12210-019-00859-9 MAR 2020
Abstract:
The optical behavior of both synthetic and natural icosahedral and decagonal quasicrystals in the range 400-700 nm has been explored by repeated measurements of the reflectance values on crystallographically oriented fragments. We found differences in the optical behavior between natural and synthetic quasicrystals, and discuss the possible reasons to justify this feature. Natural quasicrystals, both icosahedral and decagonal, exhibit an almost identical chemical composition to their synthetic analogs, but seem to show a higher degree of structural perfection, as indicated from their electron and X-ray diffraction patterns. Such perfection is mainly due to the very low phason strain shown by the minerals with respect to the synthetic compounds and substantiates the differences in the measured reflectance values. The new finding constitutes significant new support for the original proposal that icosahedral and decagonal quasicrystals can be energetically stable states of matter, on the same footing as crystals.
Title:
Low temperature magnetic properties of the quasicrystalline Al80Mn12Si4X4 (X = Cr, Y, Sm and Nd) alloys
J NON-CRYST SOLIDS 547 10.1016/j.jnoncrysol.2020.120296 NOV 1 2020
Abstract:
Quasicrystalline Al80Mn12Si4X4 (X = Cr, Y, Sm and Nd) alloys were produced and their magnetic properties were investigated. X-ray diffraction and scanning electron microscopy revealed the presence of atomic arrangements with pentagonal symmetry. The nature of the coupling of the magnetic moments was investigated for temperatures (T) varying from 2.0 to 300.0 K and for applied magnetic fields in the range +/- 85.0 kOe. The dc magnetic susceptibility indicated that the net magnetic interactions are antiferromagnetic for the samples with Cr and Y. However, the samples with Sm and Nd showed to be ferromagnetic at low-T. The coercivity and the remanence determined from hysteresis loops at 5 K for the sample with Sm (Nd) was, respectively, 0.54 kOe (0.40 kOe) and 0.047 emu/g (0.19 emu/g). A spin-glass-like phase was observed for the AlMnSiNd (AlMnSiSm) sample below 2.7 K (2.4 K). The results are accounted for by taking into consideration the coupling among the rare earth magnetic moments.
Title:
Pseudo-2-Fold Surface of the Al13Co4 Catalyst: Structure, Stability, and Hydrogen Adsorption
Authors:
Chatelier, C, Garreau, Y, Vlad, A, Ledieu, J, Resta, A, Fournee, V, De Weerd, MC, Coati, A, Gaudry, M
Source:
ACS APPL MATER INTER 12 (35):39787 10.1021/acsami.0c09702 SEP 2 2020
Abstract:
A few low-order approximants to decagonal quasicrystals have been shown to provide excellent activity and selectivity for the hydrogenation of alkenes and alkynes. It is the case for the Al13Co4 compound, for which the catalytic properties of the pseudo-2-fold orientation have been revealed to be among the best. A combination of surface science studies, including surface X-ray diffraction, and calculations based on density functional theory is used here to derive an atomistic model for the pseudo-2-fold o-Al13Co4 surface, whose faceted and columnar structure is found very similar to the one of the 2-fold surface of the d-Al-Ni-Co quasicrystal. Facets substantially stabilize the system, with energies in the range 1.19-1.31 J/m(2), i.e., much smaller than the ones of the pseudo-10-fold (1.49-1.68 J/m(2)) and pseudo-2-fold (1.66 J/m(2)) surfaces. Faceting is also a main factor at the origin of the Al13Co4 catalytic performances, as illustrated by the comparison of the pseudo-10-fold, pseudo-2-fold and facet potential energy maps for hydrogen adsorption. This work gives insights toward the design of complex intermetallic catalysts through surface nanostructuration for optimized catalytic performances.
Update: 1-Oct-2020
Title:
Thermally driven commensurate-incommensurate transition tracked by magnetochromism in chiral polar Ni2In0.9Cr0.1 SbO6
Authors:
Sato, T, Abe, N, Araki, Y, Kimura, S, Tokunaga, Y, Arima, TH
Source:
PHYS REV B 102 (9) 10.1103/PhysRevB.102.094418 SEP 15 2020
Abstract:
Ni2In0.9Cr0.1SbO6 is a trigonal chiral polar helimagnet with multiferroic nature. Here, we perform optical absorption measurements on Ni2In0.9Cr0.1SbO6 in magnetic fields. The observed magnetochromism is attributed to the spin-dependent transition probability of the intra-atomic d-d excitation from the (3)A(2g) ground state to the E-1(g) excited state of Ni2+ ions with the 3d(8) electronic configuration. Exploiting the magnetodielectric coupling in optical frequencies, we propose that the boundary between ferrimagnetic and incommensurate magnetic phases of Ni(2)In(0.9)Cr(0)(.1)SbO(6 )in 6 T along the c axis should lie at around 50 K.
Title:
Synthesis of fine skeletal structure on Al-Cu-Co decagonal quasicrystals for hydrogen production through steam reforming of methanol
Authors:
Mishra, SS, Yadav, TP, Mukhopadhyay, NK, Srivastava, ON
Source:
INT J HYDROGEN ENERG 45 (46):24491 10.1016/j.ijhydene.2020.06.216 SEP 21 2020
Abstract:
In the present investigation, we have reported the surface-microstructure, chemical composition and structural characteristics of the conventionally solidified Al65Cu15Co20 and Al65Cu20Co15 decagonal quasicrystals before and after chemical leaching treatment. The surface of the polycrystalline Al-Cu-Co decagonal phase was leached with 2.5 mol of Na2CO3 solution for the duration of 0.5 h, 2 h and 8 h. The leached surface was characterized by x-ray diffraction, scanning and transmission electron microscopy techniques. The energy dispersive x-ray analysis was employed to determine the chemical composition of the leached surface. After leaching treatment, the crystallographic structure of the leached surface was found to change from the quasicrystalline phase to the crystalline phases of Cu, Co and their oxides. The formation of the skeletal structure observed at the surface was attributed to the removal of Al atoms from the lattice points of the concerned decagonal quasilattice. This skeletal structure was found to contain porosities coexisting with finely distributed nano-particles of Cu, Co and Cu2O with sizes ranging from 10 to 28 nm. The 8 h leached surface containing these skeletal features has demonstrated excellent catalytic activity for steam reforming of methanol and it has led to hydrogen production at the rate of similar to 200 ml/g min at the reaction temperature of 580 K. (c) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Update: 24-Sep-2020
Title:
On the Possibility of Observing the Incommensurately Modulated Modification of Li2B4O7
Single crystal studies of the Li(2)B(4)O(7)borate in a wide temperature range are performed. The studies confirm the smoothed nature of the temperature dependences of theccell parameter and do not reveal any modulated phases at low temperature.
Title:
Inflation versus projection sets in aperiodic systems: the role of the window in averaging and diffraction
Authors:
Baake, M, Grimm, U
Source:
ACTA CRYSTALLOGR A 76:559 10.1107/S2053273320007421 SEP 2020
Abstract:
Tilings based on the cut-and-project method are key model systems for the description of aperiodic solids. Typically, quantities of interest in crystallography involve averaging over large patches, and are well defined only in the infinite-volume limit. In particular, this is the case for autocorrelation and diffraction measures. For cut-and-project systems, the averaging can conveniently be transferred to internal space, which means dealing with the corresponding windows. In this topical review, this is illustrated by the example of averaged shelling numbers for the Fibonacci tiling, and the standard approach to the diffraction for this example is recapitulated. Further, recent developments are discussed for cut-and-project structures with an inflation symmetry, which are based on an internal counterpart of the renormalization cocycle. Finally, a brief review is given of the notion of hyperuniformity, which has recently gained popularity, and its application to aperiodic structures.
Title:
Emergent Spin Glass Behavior Created by Self-Assembled Antiferromagnetic NiO Columns in Ferrimagnetic NiFe2O4
Authors:
Nepal, R, Wang, Z, Dai, S, Saghayezhian, M, Zhu, YM, Plummer, EW, Jin, RY
Source:
ACS APPL MATER INTER 12 (34):38788 10.1021/acsami.0c10790 AUG 26 2020
Abstract:
Spin glass (SG) is a magnetic state with spin structure incommensurate with lattice and charge. Fundamental understanding of its behavior has a profound impact on many technological problems. Here, we present a novel case of interface-induced spin glass behavior via self-assembly of single-crystalline NiO microcolumns in a single-crystalline NiFe2O4 matrix. Scanning transmission electron microscopy indicates that the hexagonal-shaped NiO columns are along their [211] direction and oriented along the [111] direction of the NiFe2O4 matrix. Magnetic force microscopy reveals magnetic anisotropy between NiO columns (antiferromagnetic transition temperature T-N similar to 523 K) and NiFe2O4 matrix (ferrimagnetic transition temperature T-FI similar to 860 K). This leads to spin disorder/frustration at atomically sharp NiFe2O4/NiO interfaces responsible for spin glass behavior below T-SG similar to 28 K. Our results demonstrate that self-assembly of magnetically distinct microstructures into another crystalline and magnetically ordered matrix is an effective way to create novel spin states at interfaces.
Title:
Pressure-induced incommensurate antiferromagnetic order in a ferromagnetic B-site ordered double-perovskite Lu2NiMnO6
Authors:
Terada, N, Colin, CV, Qureshi, N, Hansen, TC, Matsubayashi, K, Uwatoko, Y, Belik, AA
Source:
PHYS REV B 102 (9) 10.1103/PhysRevB.102.094412 SEP 9 2020
Abstract:
We have investigated the pressure effect on magnetic ordering of the ferromagnetic double perovskite Lu2NiMnO6 by magnetization, ac magnetic susceptibility, and neutron diffraction experiments up to 8.0 GPa in order to understand the ferromagnetic-to-antiferromagnetic phase transition by substitution of the A sites in A(2)NiMnO(6) from rare-earth to indium or scandium ions. Strong ferromagnetic spin correlation seen in the susceptibility at low pressure is significantly suppressed by increasing pressure. In a neutron diffraction experiment, the magnetic Bragg reflections associated with ferromagnetic ordering disappear above 4.5 GPa. For the high-pressure region above 4.5 GPa, an antiferromagnetic ordering with long-period incommensurate modulation appears, which is coexistent with ferromagnetic short-range order. From mean-field calculations, we infer that pressure modification of the delicate balance of the competing next-nearest-neighbor exchange interactions between Ni and Ni, or Mn and Mn, plays an important role in the phase transition from ferromagnetic to antiferromagnetic ordering in A(2)NiMnO(6).
Title:
Formation Mechanism of the Helical Q Structure in Gd-Based Skyrmion Materials
Authors:
Nomoto, T, Koretsune, T, Arita, R
Source:
PHYS REV LETT 125 (11) 10.1103/PhysRevLett.125.117204 SEP 9 2020
Abstract:
Using the ab initio local force method, we investigate the formation mechanism of the helical spin structure in GdRu2Si2 and Gd2PdSi3. We calculate the paramagnetic spin susceptibility and find that the Fermi surface nesting is not the origin of the incommensurate modulation, in contrast to the naive scenario based on the Ruderman-Kittel-Kasuya-Yosida mechanism. We then decompose the exchange interactions between the Gd spins into each orbital component, and show that spin-density-wave type interaction between the Gd-5d orbitals is ferromagnetic, but the interaction between the Gd-4f orbitals is antiferromagnetic. We conclude that the competition of these two interactions, namely, the interorbital frustration, stabilizes the finite-Q structure.
Title:
Instability and evolution of the magnetic ground state in metallic perovskites GdRh3C1-xBx
Authors:
Pandey, A, Singh, AK, Dan, S, Ghosh, K, Das, I, Tripathi, S, Kumar, U, Ranganathan, R, Johnston, DC, Mazumdar, C
Source:
PHYS REV MATER 4 (8) 10.1103/PhysRevMaterials.4.084411 AUG 31 2020
Abstract:
We report investigations of the structural, magnetic, electrical transport, and thermal properties of five compositions of the metallic perovskite GdRh3C1-xBx (0.00 <= x <= 1.00). Our results show that all five compositions undergo magnetic ordering at low temperatures, but the nature of the ordered state is significantly different in the carbon- and the boron-rich compositions, where the former shows signatures of an amplitude-modulated magnetic structure and the latter exhibits evidence of an equal-moment incommensurate antiferromagnetic ordering. We also observe a remarkable field-dependent evolution of conduction carrier polarization in the compositionally disordered compounds. The outcomes indicate that this system is energetically situated in proximity to a magnetic instability where small variations in the control parameter(s), such as the lattice constant and/or electron density, lead to considerably different ground states.
Title:
Presaturation phase in the frustrated ferro-antiferromagnet Pb2VO(PO4)(2)
Authors:
Landolt, F, Bettler, S, Yan, Z, Gvasaliya, S, Zheludev, A, Mishra, S, Sheikin, I, Kramer, S, Horvatic, M, Gazizulina, A, Prokhnenko, O
Source:
PHYS REV B 102 (9) 10.1103/PhysRevB.102.094414 SEP 10 2020
Abstract:
Magnetization, magnetic torque, neutron diffraction and NMR experiments are used to map out the H - T phase diagram of the prototypical quasi-two-dimensional ferro-antiferromagnet Pb2VO(PO4)(2) in magnetic fields up to 27 T. When the field is applied perpendicularly to the axis of magnetic anisotropy, a new magnetic state emerges through a discontinuous transition and persists in a narrow field range just below saturation. The measured NMR spectra suggest a complex and possibly incommensurate magnetic order in that regime.
Title:
Structural-transition-driven antiferromagnetic to spin-glass transition in Cd-Mg-Tb 1/1 approximants
Authors:
Labib, F, Okuyama, D, Fujita, N, Yamada, T, Ohhashi, S, Morikawa, D, Tsuda, K, Sato, TJ, Tsai, AP
Source:
J PHYS-CONDENS MAT 32 (48) 10.1088/1361-648X/aba921 NOV 18 2020
Abstract:
The magnetic susceptibility of the 1/1 approximants to icosahedral quasicrystals in a series of Cd85-xMgxTb15(x= 5, 10, 15, 20) alloys was investigated in detail. The occurrence of antiferromagnetic (AFM) to spin-glass (SG)-like transition was noticed by increasing Mg. Transmission electron microscopy analysis evidenced a correlation between the magnetic transition and suppression of the monoclinic superlattice ordering with respect to the orientation of the Cd(4)tetrahedron atT> 100 K. The possible origins of this phenomenon were discussed in detail. The occurrence of the AFM to SG-like magnetic transition is associated with the combination of chemical disorder due to a randomized substitution of Cd with Mg and the orientational disorder of the Cd(4)tetrahedra.
Title:
Symmetry and Asymmetry in Quasicrystals or Amorphous Materials
Authors:
Barber, EM
Source:
SYMMETRY-BASEL 12 (8) 10.3390/sym12081326 AUG 2020
Abstract:
Quasicrystals (QCs) are long-range ordered materials with a symmetry incompatible with translation invariance. Accordingly, QCs exhibit high-quality diffraction patterns containing a collection of discrete Bragg reflections. Notwithstanding this, it is still common to read in the recent literature that these materials occupy an intermediate position between amorphous materials and periodic crystals. This misleading terminology can be understood as probably arising from the use of models and notions borrowed from the amorphous solid's conceptual framework (such us tunneling states, weak interference effects, variable range hopping, or spin glass) in order to explain certain physical properties observed in QCs. On the other hand, the absence of a general, full-fledged theory of quasiperiodic systems certainly makes it difficult to clearly distinguish the features related to short-range order atomic arrangements from those stemming from long-range order correlations.
Title:
Discrete Block Copolymers with Diverse Architectures: Resolving Complex Spherical Phases with One Monomer Resolution
ACS CENTRAL SCI 6 (8):1386 10.1021/acscentsci.0c00798 AUG 26 2020
Abstract:
This work describes the first rigorous example of a single-component block copolymer system forming unconventional spherical phases. A library of discrete block polymers with uniform chain length and diverse architectures were modularly prepared through a combination of a step-growth approach and highly efficient coupling reactions. The precise chemical structure eliminates all the molecular defects associated with molar weight, dispersity, and compositional ratio. Complex spherical phases, including the Frank-Kasper phase (A15 and sigma) and quasicrystalline phase, were experimentally captured by meticulously tuning the composition and architectures. A phase portrait with unprecedented accuracy was mapped out (up to one monomer resolution), unraveling intriguing details of phase behaviors that have long been compromised by inherent molecular weight distribution. This study serves as a delicate model system to bridge the existing gaps between experimental observations and theoretical assessments and to provide insights into the formation and evolution of the unconventional spherical phases in soft matter systems.
Title:
Design of quasicrystal alloys with favorable tribological performance in view of microstructure and mechanical properties
Authors:
Lee, KJ, Chen, Y, Dai, W, Naugle, D, Liang, H
Source:
MATER DESIGN 193 10.1016/j.matdes.2020.108735 AUG 2020
Abstract:
Quasicrystals have been used in various applications to improve wear resistance as well as friction. It is known that quasicrystal (i-phase) content and microstructure in alloys have a decisive effect on the mechanical properties and tribological performance. In this research, four (beta + i)-dual-phased quasicrystal alloys with different i-phase content and grain size were developed to alleviate the brittleness of the i-phase with the help of the soft beta-phase. The influences of the i-phase content and grain size were investigated through impact test, wear test, and analysis. Through the annealing process, the amount of the i-phase was increased by about 38% (59.24% -> 81.75%), and, besides, the grain size of the i-phase was simultaneously increased from 3.47 mu m up to 9.98 mu m. As the amount of i-phase increased, it was possible to increase the hardness from 712 HV to 763 HV. Meanwhile, the increased grain size (i-phase) reduced the contact stress of the grain during wear testing: thus, the specific wear rate could be decreased from 2.21 x 10(-4) mm(3)/Nm to 0.5 x 10(-4) mm(3)/Nm. Not only that, but an experimental wear equation was obtained using empirical data to predict the wear behavior of the dual-phased alloys. (C) 2020 Published by Elsevier Ltd.
Update: 17-Sep-2020
Title:
A Layered Tin Bismuth Selenide with Three Different Building Blocks that Account for an Extremely Large Lattice Parameter of 283 angstrom
Authors:
Nentwig, M, Eisenburger, L, Heinke, F, Souchay, D, Oeckler, O
Source:
CHEM-EUR J 26 (47):10676 10.1002/chem.202000663 AUG 21 2020
Abstract:
The layered compound Sn(2.8(4))Bi(20.2(4))Se(27)exhibits an extraordinarily long-periodic 150Rstacking sequence. The crystal structure contains three different building blocks, which form upon the addition of Sn to a Bi-rich bismuth selenide. Sn-doped Bi(2)double ("2") layers similar to those in elemental bismuth, Sn(0.3)Bi(1.7)Se(3)quintuple ("5") layers and Sn(0.4)Bi(2.6)Se(4)septuple ("7") layers are arranged in a 7525757525|7525757525|7525757525 sequence, which corresponds to a structure witha=4.1819(4) andc=282.64(6) angstrom in space groupR3?m. The structure of a microcrystal was determined using microfocused synchrotron radiation and refined as a formally commensurately modulated structure in (3+1)D superspace (superspace groupR3?m(00 gamma)00), with a trivial basic structure that contains just one atom. The stacking sequence as well as the cation distribution are confirmed by aberration-corrected scanning transmission electron microscopy (STEM) in combination with chemical mapping by X-ray spectroscopy with atomic resolution. Stacking faults are not typical but have been observed occasionally.
Title:
Charge density waves beyond the Pauli paramagnetic limit in 2D systems
Authors:
Aperis, A, Varelogiannis, G
Source:
J APPL PHYS 128 (8) 10.1063/5.0015993 AUG 28 2020
Abstract:
Two-dimensional materials are ideal candidates to host Charge Density Waves (CDWs) that exhibit paramagnetic limiting behavior, similar to the well-known case of superconductors. Here, we study how CDWs in two-dimensional systems can survive beyond the Pauli limit when they are subjected to a strong magnetic field by developing a generalized mean-field theory of CDWs under Zeeman fields that includes incommensurability, imperfect nesting, and temperature effects and the possibility of a competing or coexisting Spin Density Wave (SDW) order. Our numerical calculations yield rich phase diagrams with distinct high-field phases above the Pauli limiting field. For perfectly nested commensurate CDWs, a q-modulated CDW phase that is completely analogous to the superconducting Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase appears at high fields. In the more common case of imperfect nesting, the commensurate CDW ground state undergoes a series of magnetic-field-induced phase transitions first into a phase where commensurate CDW and SDW coexist and subsequently into another phase where CDW and SDW acquire a q-modulation that is, however, distinct from the pure FFLO CDW phase. The commensurate CDW+SDW phase occurs for fields comparable to but less than the Pauli limit and survives above it. Thus, this phase provides a plausible mechanism for the CDW to survive at high fields without the need for forming the more fragile FFLO phase. We suggest that the recently discovered 2D materials like the transition metal dichalcogenides offer a promising platform for observing such exotic field-induced CDW phenomena.
Title:
(X, T)-phase diagram of TlIn(S1-xSex)(2)solid solutions (x =0, 0.02, 0.06, 0.10, 0.15 and 0.25). Temperature dependences of thermal expansion and optical anisotropy parameters
Authors:
Adamenko, D, Say, A, Martynyuk-Lototska, I, Mys, O, Kostyrko, M, Gommonnai, OO, Gomonnai, AV, Vlokh, R
Basing on dilatometric and optical anisotropy studies, we have obtained (x, T)-phase diagram for TlIn(S1-xSex)(2)solid solutions in the range 0 <= x <=( )0.25 of selenium concentrations. It has been found that increasing selenium concentration leads to decrease in almost all of phase transition (PT) temperatures. At the same time, the temperatureT(i)of PT into incommensurate phase for the polytypec = c(0)increases at the selenium concentrations higher than 0.1, and we observe a tendency to broadening of the temperature region where the incommensurate phase exists. The incommensurate-ferroelectric PT reveals a notably diffuse character. We have also demonstrated that the TlIn(S1-xSex)(2)solid solutions have a polytype structure. It manifests itself in availability of additional PT in the solid solutions with 0 <= x <=( )0.05. Most probably, the latter corresponds to ferroelectric transformation in the polytype withc = 16c(0).
Title:
Physical properties of weak-coupling quasiperiodic superconductors
Authors:
Takemori, N, Arita, R, Sakai, S
Source:
PHYS REV B 102 (11) 10.1103/PhysRevB.102.115108 SEP 3 2020
Abstract:
We numerically study the physical properties of quasiperiodic superconductors with the aim of understanding superconductivity in quasicrystals. Considering the attractive Hubbard model on the Penrose tiling as a simple theoretical model, we calculate various basic superconducting properties and find deviations from the universal values of the Bardeen-Cooper-Schrieffer theory with a constant density-of-states approximation. In particular, we find that the jump of the specific heat at the superconducting transition is about 10%-20% smaller than that universal value, consistent with the experimental results obtained for the superconducting Al-Mg-Zn quasicrystalline alloy. Furthermore, we calculate current-voltage characteristics and find that the current gradually increases with the voltage on the Penrose tiling in contrast to a rapid increase in the periodic system. These distinctions originate from the nontrivial Cooper pairing characteristic of the quasiperiodic system.
Title:
Quasicrystalline phase-change memory
Authors:
Lee, ES, Yoo, JE, Yoon, DS, Kim, SD, Kim, Y, Hwang, S, Kim, D, Jeong, HC, Kim, WT, Chang, HJ, Suh, H, Ko, DH, Cho, C, Choi, Y, Kim, DH, Cho, MH
Source:
SCI REP-UK 10 (1) 10.1038/s41598-020-70662-2 AUG 13 2020
Abstract:
Phase-change memory utilizing amorphous-to-crystalline phase-change processes for reset-to-set operation as a nonvolatile memory has been recently commercialized as a storage class memory. Unfortunately, designing new phase-change materials (PCMs) with low phase-change energy and sufficient thermal stability is difficult because phase-change energy and thermal stability decrease simultaneously as the amorphous phase destabilizes. This issue arising from the trade-off relationship between stability and energy consumption can be solved by reducing the entropic loss of phase-change energy as apparent in crystalline-to-crystalline phase-change process of a GeTe/Sb2Te3 superlattice structure. A paradigm shift in atomic crystallography has been recently produced using a quasi-crystal, which is a new type of atomic ordering symmetry without any linear translational symmetry. This paper introduces a novel class of PCMs based on a quasicrystalline-to-approximant crystalline phase-change process, whose phase-change energy and thermal stability are simultaneously enhanced compared to those of the GeTe/Sb2Te3 superlattice structure. This report includes a new concept that reduces entropic loss using a quasicrystalline state and takes the first step in the development of new PCMs with significantly low phase-change energy and considerably high thermal stability.
Title:
Solid state interactions in the La-Au-Mg system: phase equilibria, novel compounds and chemical bonding
Authors:
Freccero, R, De Negri, S, Saccone, A, Solokha, P
Source:
DALTON T 49 (34):12056 10.1039/d0dt02359k SEP 14 2020
Abstract:
Gold intermetallic chemistry is very rich, covering different classes of compounds ranging from the Hume-Rothery to Zintl phases to polar intermetallics to quasicrystals. Au's relativistic effects are frequently mentioned as responsible for the peculiar structural and physical properties of its compounds, nonetheless the aspects of chemical bonding are far to be clearly understood. In this work, the La-Au-Mg system was targeted for the discovery of new gold intermetallics and their structural and chemical bonding characterization. Studies on solid state interactions resulted in the construction of a partial La-Au-Mg isothermal section at 400 degrees C. The high reactivity between the constituents is reflected by the formation of five intermetallic compounds in the concentration range of less than 50 at% of Au. A complete crystallographic study was conducted for four of them, namely La1.82Au3+xMg14.36-x (0 <= x <= 0.90, hP42-3.64-CeMg10.3), La3Au4-xMg12+x (0 <= x <= 0.75, hP38-Gd3Ru4Al12), LaAuMg2 (oS16-MgCuAl2) and LaAu1+xMg1-x (0 <= x <= 0.15, hP9-ZrNiAl). A unifying description based on the different stacking sequences of equal slabs along the c-axis is proposed for these intermetallics. Chemical bonding in LaAuMg2 was studied by following the position space approach and including relativistic effects. Among the peculiarities of this LaMg2Au auride, there are two-atomic La-Au bonds showing a classical polar covalent character and that form distorted hexagonal planar layers and multi-atomic bonds involving Mg species. One of these is interpreted as a Mg-Mg bond supported by the neighbouring La and Au atoms, explaining the Mg reduced oxidation state (close to +1) in this compound.
Title:
Supramolecular Self-Assembly of Perylene Bisimide-Based Rigid Giant Tetrahedra
Recently, ordered structures constructed from rigid three-dimensional (3D) shaped polyhedra have been drawing general interest, with the tetrahedron being the simplest one but showing complicated assembly behaviors. Rigid tetrahedron building blocks have been shown to form quasicrystalline and crystalline phases with high packing fractions by both simulation and experiments. Nevertheless, the study of 3D tetrahedral building blocks is limited, especially in the field of supramolecular self-assembly. Here, we present an experimental study of rigid giant tetrahedral molecules constructed by attaching four bulky polyhedral oligomeric silsesquioxane (POSS) cages to a tetrahedral perylene bisimide (PBI) scaffold. Self-assembly of these giant tetrahedra is mediated by pi-pi interaction between the tetrahedral PBI-based scaffolds and their overall tetrahedral symmetry. A monolithic nearly centimeter-sized hexagonal supramolecular structure was observed in the giant tetrahedron with short flexible linkers between PBI and POSS cages, while a micrometer-sized crystalline helical structure formed in that with completely rigid aromatic linkers. Their significant difference in electrical conductivity could be explained by two completely different packing models of the giant tetrahedra.
Update: 10-Sep-2020
Title:
Role of "Dumbbell" Pairs of Fe in Spin Alignments and Negative Thermal Expansion of Lu2Fe17-Based Intermetallic Compounds
INORG CHEM 59 (16):11228 10.1021/acs.inorgchem.0c01590 AUG 17 2020
Abstract:
Knowledge of negative thermal expansion (NTE) is an interesting issue in the field of materials science and engineering. It has been proposed that the unique dumbbell pairs of Fe (dumbbells) are highly entangled in the NTE behaviors of R2Fe17 (R = rare earth) compounds but still remain controversial. Here, a facile method is employed to explore the role of dumbbells in spin alignments and NTE by the nonstoichiometric design of Lu2-xFe17 compounds. The powder synchrotron X-ray diffraction, magnetometry, and neutron powder diffraction investigations indicate that a decrease of the Lu content can enhance the dumbbell concentration and motivate an incommensurate magnetic structure simultaneously. However, increasing the dumbbell concentration makes little difference in the amplitude of the ordered magnetic moments of Fe sublattices, which reveals an equivalent NTE behavior for Lu2-xFe17 compounds. This work gives insight into the role that dumbbells played in spin alignments and NTE for Lu2Fe17-based compounds, correcting the previously proposed conjecture and probably conducive to adjusting the related magnetic performances of R2Fe17 compounds in the future.
Title:
Magnetic phase transitions in Eu(Co1-xNix)(2-y)As-2 single crystals
PHYS REV MATER 4 (8) 10.1103/PhysRevMaterials.4.084407 AUG 12 2020
Abstract:
The effects of Ni doping in Eu(Co1-xNix)(2-y)As-2 single crystals with x = 0 to 1 grown out of self-flux are investigated via crystallographic, electronic transport, magnetic, and thermal measurements. All compositions adopt the body-centered-tetragonal ThCr2Si2 structure with space group I4/mmm. We also find 3%-4% of randomly distributed vacancies on the Co/Ni site. Anisotropic magnetic susceptibility chi(alpha) (alpha = ab, c) data versus temperature T show clear signatures of an antiferromagnetic (AFM) c-axis helix structure associated with the Eu+2 spins 7/2 for x = 0 and 1 as previously reported. The chi(alpha)(T) data for x = 0.03 and 0.10 suggest an anomalous 2q magnetic structure containing two helix axes along the c axis and in the ab plane, respectively, whereas for x = 0.75 and 0.82 a c-axis helix is inferred as previously found for x = 0 and 1. At intermediate compositions x = 0.2, 0.32, 0.42, 0.54, and 0.65, a magnetic structure with a large ferromagnetic (FM) c-axis component is found from magnetization versus field isotherms, suggested to be an incommensurate FM c-axis cone structure associated with the Eu spins, which consists of both AFM and FM components. In addition, the chi(T) and heat capacity C-p(T) data for x = 0.2-0.65 indicate the occurrence of itinerant FM order associated with the Co/Ni atoms with Curie temperatures from 60 to 25 K, respectively. Electrical resistivity rho(T) measurements indicate metallic character for all compositions with abrupt increases in slope on cooling below the Eu AFM transition temperatures. In addition to this panoply of magnetic transitions, Eu-151 Mossbauer measurements indicate that ordering of the Eu moments proceeds via an incommensurate sine amplitude-modulated structure with additional transition temperatures associated with this effect.
Title:
Fingerprints of spin-current physics on magnetoelectric response in the spin-1/2 magnet Ba2CuGe2O7
Authors:
Ono, R, Nikolaev, S, Solovyev, I
Source:
PHYS REV B 102 (6) 10.1103/PhysRevB.102.064422 AUG 24 2020
Abstract:
As is well known, the single-site anisotropy vanishes in the spin-1/2 compounds as a consequence of the fundamental Kramers degeneracy. We argue, rather generally, that a similar property holds for the magnetically induced electric polarization P, which should depend only on the relative orientation of spins in the bonds but not on the direction of each individual spin. Thus, for insulating multiferroic compounds, P can be decomposed in terms of pairwise isotropic, antisymmetric, and anisotropic symmetric contributions, which can be rigorously derived in the framework of the superexchange (SE) theory, in analogy with the spin Hamiltonian. The SE theory also allows us to identify the microscopic mechanism that stands behind each contribution. The most controversial and intriguing one-concerning the form, appearances, and implications for the properties of real compounds-is the antisymmetric or spin-current mechanism. In this work, we propose that, within the SE theory, the disputed magnetoelectric (ME) properties of tetragonal Ba2CuGe2O7, representing the lattice of magnetic Cu2+ ions in the tetrahedral environment, can be explained solely by the spin-current mechanism, while other contributions are either small or forbidden by symmetry. First, after analysis of the symmetry properties of the SE Hamiltonian and corresponding parameters of electric polarization, we explicitly show how the cycloidal spin order induces the experimentally observed electric polarization in the direction perpendicular to the tetragonal plane, which can be naturally explained by the spin-current mechanism operating in the out-of-plane bonds. Then, we unveil the previously overlooked ME effect, where the application of the magnetic field perpendicular to the plane not only causes the incommensurate-commensurate transition, but also flips the electric polarization into the plane due to the spin-current mechanism operating in the neighboring bonds within this plane. In both cases, the magnitude and direction of P can be controlled by rotating the spin pattern in the tetragonal plane. Our analysis is based on a realistic spin model, which was rigorously derived from first-principles electronic structure calculations and supplemented with a new algorithm for the construction of localized Wannier functions obeying the crystallographic symmetry of Ba2CuGe2O7.
Title:
Identifying and characterizing translationally modulated molecular crystal structures
Authors:
Brock, CP, Taylor, R
Source:
ACTA CRYSTALLOGR B 76:630 10.1107/S2052520620007891 AUG 2020
Abstract:
Most structural (i.e. displacive) modulations make molecules independent that had been related by translation in a phase having a smaller or centered unit cell. In the modulated structure the independent molecules are differentiated by small translations, rotations, and/or conformational changes but an approximate translational relationship is normally retained. A program has been written to identify such pseudotranslations because they can be difficult to find by eye and because they combine with each other and with lattice translations in ways that can be confusing. To characterize the pseudotranslations the program calculates their fractional translational, orientational, and conformational components as well as several quality indicators. While many pseudotranslations are obvious, others are borderline; setting tolerances for identifying a pseudotranslation proved difficult. Defaults were chosen to reproduce experience-based judgment but they can be varied in the program input. The program was run for organic and for metallo-organic structures with R <= 0.075 in the 2019 release of the Cambridge Structural Database. The frequency of pseudotranslations increases with Z and is approximately 50% for Z' > 4. Some structures were found in which an identified pseudotranslation cannot correspond to a modulation. These include structures in which some but not all of the molecules are related by pseudotranslations and structures in which pseudotranslations in different parts of the unit cell have different directions.
Title:
Spatial ordering of the charge density waves in NbSe3
Authors:
van Midden, MA, van Midden, HJP, Prodan, A, Bennett, JC, Zupanic, E
Source:
PHYS REV B 102 (7) 10.1103/PhysRevB.102.075442 AUG 25 2020
Abstract:
Ordering of the two incommensurate charge density waves (CDW), q(1) = (0.0, 0.243, 0.0) and q(2) = (0.5, 0.263, 0.5), in the quasi-one-dimensional NbSe3 structure is studied by means of low-temperature scanning tunneling microscopy. Larger (100) van der Waals surfaces are analyzed using one-dimensional Fourier transforms along the three types of trigonal prismatic columns, running parallel to the monoclinic b0 direction. The procedure enables unambiguous differentiation between both CDWs modulating individual columns. In addition, it allows a quantitative comparison of modulation amplitudes along different columns of the same type and, in case of sufficiently large images, also along individual columns. It is confirmed that each CDW modulates both type-III and type-I columns with the narrowest isosceles bases. As a consequence beating is observed between the two slightly different modes. The bridging type-II columns appear not to be modulated. On a large scale the modulation amplitudes along individual columns vary, suggesting formation of CDW nanodomains. The possibility of interchanging both CDWs along columns forming symmetry-related pairs results in a charge difference, which is supposed to be the possible origin of CDW sliding.
Title:
Effect of doping concentration on Gd1-xAlxMnO3 structure and magnetic properties
Authors:
Rasras, A, Hamdi, R, Mansour, S, Samara, A, Haik, Y
Source:
J MAGN MAGN MATER 513 10.1016/j.jmmm.2020.167009 NOV 1 2020
Abstract:
Gd1-xAlxMnO3 material was successfully synthesized using the Pechini-modified-sol-gel method at different Al doping concentrations (x = 0, 0.25, 0.75, and 1). The samples were structurally characterized using XRD patterns and SEM/EDS analysis. The sample x = 0 displayed a single-phase, while the other systems displayed a multi-phase structure. Particle size calculated using the SEM images confirmed a successful synthesis of nanomaterials in the range of 97 and 123 nm. FC and ZFC magnetization modes were conducted for the samples, and we noticed a gradual decrease in the magnetization values as the doping increases. However, the doping also resulted in shifting the bifurcation temperature to elevated ones. The compound with x = 1 showed a good increase in magnetization up to around 8 emu/g with a bifurcation temperature of 68 K opening the door for different sets of applications at higher temperatures. The three samples (x = 0, 0.25, and 0.75) present different magnetic phases while decreasing the temperature starting from the paramagnetic phase at room temperature to the antiferromagnetic phase at the bifurcation temperature T-irr, then to the superparamagnetic behavior at very low temperatures T-B. While for the sample x = 1 illustrates only two magnetic phases; from the paramagnetic order to the antiferromagnetic phase at T-irr. Griffiths phase was also inspected for different doping concentrations, and a proportional relation was drawn between the Griffiths regime and the aluminum percentage. The isothermal entropy measurements were conducted in a magnetic field between 1 and 7 T. The concentration of aluminum in the samples reduces the produced entropy change but widens the temperature difference. The x = 0 sample shows the highest isothermal entropy change and RCP of 50 J.Kg(-1).K-1 and 552 J/kg, respectively, at 7 T. Also, the sample with x = 1 showed a maximum entropy change at 50 K due to the formation of an incommensurate antiferromagnetic phase.
Title:
Perpendicular space accounting of localized states in a quasicrystal
Authors:
Mirzhalilov, M, Oktel, MO
Source:
PHYS REV B 102 (6) 10.1103/PhysRevB.102.064213 AUG 26 2020
Abstract:
Quasicrystals can be described as projections of sections of higher dimensional periodic lattices into real space. The image of the lattice points in the projected-out dimensions, called the perpendicular space, carries valuable information about the local structure of the real space lattice. In this paper, we use perpendicular space projections to analyze the elementary excitations of a quasicrystal. In particular, we consider the vertex tight-binding model on the two-dimensional Penrose lattice and investigate the properties of strictly localized states using their perpendicular space images. Our method reproduces the previously reported frequencies for the six types of localized states in this model. We also calculate the overlaps between different localized states and show that the number of type-five and type-six localized states which are independent from the four other types is a factor of golden ratio tau = (1 + root 5)/2 higher than previously reported values. Two orientations of the same type-five or type-six which are supported around the same site are shown to be linearly dependent with the addition of other types. We also show through exhaustion of all lattice sites in perpendicular space that any point in the Penrose lattice is either in the support of at least one localized state or is forbidden by local geometry to host a strictly localized state.
Update: 3-Sep-2020
Title:
Multivalley Free Energy Landscape and the Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX2 Compounds
Authors:
Nakatsugawa, K, Tanda, S, Ikeda, TN
Source:
SCI REP-UK 10 (1) 10.1038/s41598-020-58013-7 JAN 27 2020
Abstract:
Ultrathin sheets of transition metal dichalcogenides (MX2) with charge density waves (CDWs) is increasingly gaining interest as a promising candidate for graphene-like devices. Although experimental data including stripe/quasi-stripe structure and hidden states have been reported, the ground state of ultrathin MX2 compounds and, in particular, the origin of anisotropic (stripe and quasi-stripe) CDW phases is a long-standing problem. Anisotropic CDW phases have been explained by Coulomb interaction between domain walls and inter-layer interaction. However, these models assume that anisotropic domain walls can exist in the first place. Here, we report that anisotropic CDW domain walls can appear naturally without assuming anisotropic interactions: We explain the origin of these phases by topological defect theory (line defects in a two-dimensional plane) and interference between harmonics of macroscopic CDW wave functions. We revisit the McMillan-Nakanishi-Shiba model for monolayer 1T-TaS2 and 2H-TaSe2 and show that CDWs with wave vectors that are separated by 120 degrees (i.e. the three-fold rotation symmetry of the underlying lattice) contain a free-energy landscape with many local minima. Then, we remove this 120 degrees constraint and show that free energy local minima corresponding to the stripe and quasi-stripe phases appear. Our results imply that Coulomb interaction between domain walls and inter-layer interaction may be secondary factors for the appearance of stripe and quasi-stripe CDW phases. Furthermore, this model explains our recent experimental result (appearance of the quasi-stripe structure in monolayer 1T-TaS2) and can predict new CDW phases, hence it may become the basis to study CDW further. We anticipate our results to be a starting point for further study in two-dimensional physics, such as explanation of "Hidden CDW states", study the interplay between supersolid symmetry and lattice symmetry, and application to other van der Waals structures.
Title:
Origin of the large ferroelectric polarization enhancement under high pressure for multiferroic DyMnO3 studied by polarized and unpolarized neutron diffraction
Authors:
Terada, N, Qureshi, N, Stunault, A, Enderle, M, Ouladdiaf, B, Colin, CV, Khalyavin, DD, Manuel, P, Orlandi, F, Miyahara, S, Prabhakaran, D, Osakabe, T
Source:
PHYS REV B 102 (8) 10.1103/PhysRevB.102.085131 AUG 14 2020
Abstract:
The multiferroic perovskite rare earth manganites RMnO3 (R = Dy, Tb, Gd) are known as multiferroics exhibiting pressure-induced gigantic ferroelectric polarization. In this study, we have investigated the magnetic orderings in the pressure-induced phases for DyMnO3, by neutron diffraction and spherical neutron polarimetry (SNP) experiments up to 8.0 GPa. The magnetic ordering for Mn spins changes from the incommensurate bc-cycloid to the commensurate collinear E-type structure with k(Mn) = (0, 1/2, 0) above 4.0 GPa, which is concomitant with the appearance of a giant ferroelectric polarization. The magnetic ordering for the Dy spins has been determined to be a noncollinear spin structure with a and b spin components and k(Dy) = (0, 1/2, 0) for the low- and high-pressure phases. The magnetic field along the a axis, H-a, affects the Dy ordering, which is seen in the changes in the k vector from k(Dy) = (0, 1/2, 0) in H-a <= 3 T to k(Dy) = (0, 0, 0) in H-a >= 3 T. Considering the lattice distortion generated by the determined magnetic orderings through the exchange striction mechanism, we conclude that the exchange striction for rare earth and Mn bonds, which is added to the uniform polarization generated by the E-type Mn ordering, is strongly related to the significant magnetic field enhancement of ferroelectric polarization in the high-pressure phase of the rare earth manganites.
Title:
Discovery of an unconventional charge modulation on the surface of charge-density-wave material TaTe4
NEW J PHYS 22 (8) 10.1088/1367-2630/aba065 AUG 2020
Abstract:
Electronic systems with quasi-one-dimensional (Q1D) Fermi surface tend to form either a charge-density-wave (CDW) or a spin-density-wave ground state at low temperatures due to one-dimensional instabilities. Among various CDW states, surface CDWs are different from that within the bulk due to the reduced dimensionality. Here we report the systematic investigation of charge density modulation on the surface ofin situcleaved TaTe(4)crystal by means of low temperature scanning tunneling microscopy/spectroscopy, corroborated with density functional theory calculation. Well-defined Q1D modulation (4a, 6c) accompanied with a periodic lattice distortion is clearly observed on the (010) cleaved surface, distinct from that of its bulk CDW (2ax 2ax 3c). Tunneling spectroscopic measurements reveal a partially-opened energy gap about 23 meV around Fermi level. Such gap shows similar spatial variation with the periodicity of surface modulation and diminishes subsequently as temperature rises, which indicates a novel surface-related CDW gap states. The surface modulation vectors fit well with the Fermi surface nesting vectors, derived from the calculated Fermi surfaces. Surprisingly, such surface modulation can be suppressed greatly by applying vertical magnetic field and a critical field about 9.05 T can be estimated from field-dependent data. Our results demonstrate that this unique CDW modulation is strongly related to Fermi surface nesting mediated electron-electron coupling due to the reduced dimensionality of the surface, and can be readily tuned by relatively small magnetic field.
Title:
Influence of differences in orientational planar density on the growth of Pb on the i-Ag-In-Yb quasicrystal
Authors:
Coates, S, McGrath, R, Sharma, HR
Source:
J PHYS-CONDENS MAT 32 (42) 10.1088/1361-648X/aba294 OCT 7 2020
Abstract:
Metal adsorption upon the 3-fold and 5-fold symmetric surfaces of the i-Ag-In-Yb quasicrystal has led to the observation of unique growth modes. Here, we present a study of the growth of Pb upon the 2-fold i-Ag-In-Yb surface, where the growth mechanism is found to be different from those observed on the other, higher symmetry surfaces of the same system. Initial Pb atoms occupy non-chemically-specific surface sites before forming a row structure. At higher coverages, the Pb atoms form a dense wetting layer before 1D Pb chains of limited size are self-assembled as a second layer. We therefore consider the Pb atoms to exhibit a type of Stranski-Krastanov growth mode. Substrate-adsorbate interaction is favoured in the wetting layer, before adsorbate-adsorbate interaction promotes chain growth. The difference in growth modes upon the three high symmetry surfaces is discussed with respect to their respective atomic densities.
Title:
Nature of protected zero-energy states in Penrose quasicrystals
Authors:
Day-Roberts, E, Fernandes, RM, Kamenev, A
Source:
PHYS REV B 102 (6) 10.1103/PhysRevB.102.064210 AUG 20 2020
Abstract:
The electronic spectrum of the Penrose rhombus quasicrystal exhibits a macroscopic fraction of exactly degenerate zero-energy states. In contrast to other bipartite quasicrystals, such as the kite-and-dart one, these zero-energy states cannot be attributed to a global mismatch Delta n between the number of sites in the two sublattices that form the quasicrystal. Here, we argue that these zero-energy states are instead related to a local mismatch Delta n(r). Although Delta n(r) averages to 0, its staggered average over self-organized domains gives the correct number of zero-energy states. Physically, the local mismatch is related to a hidden structure of nested self-similar domains that support the zero-energy states. This allows us to develop a real-space renormalization-group scheme, which yields the scaling law for the fraction of zero-energy states, Z, versus the size of their support domain, N, as Z proportional to N-eta with eta = 1 - ln 2/2 ln tau approximate to 0.2798 (where tau is the golden ratio). It also reproduces the known total fraction of zero-energy states, 81-50 tau approximate to 0.0983. We also show that the exact degeneracy of these states is protected against a wide variety of local perturbations, such as irregular or random hopping amplitudes, magnetic field, and random dilution of the lattice. We attribute this robustness to the hidden domain structure.
Update: 27-Aug-2020
Title:
Local properties and phase transitions in Sn doped antiferroelectric PbHfO(3)single crystal
Authors:
Jankowska-Sumara, I, Pasciak, M, Kadziolka-Gawel, M, Podgorna, M, Majchrowski, A, Roleder, K
Source:
J PHYS-CONDENS MAT 32 (43) 10.1088/1361-648X/ab9bca OCT 14 2020
Abstract:
Pb(Hf1-xSnx)O(3)single crystals withx= 0.23 were characterized using single-crystal x-ray diffraction in the wide temperature range. The information on the structure of two intermediate phases, situated between low temperature antiferroelectric and high temperature paraelectric phases, has been obtained. The lower-temperature intermediate AFE2 phase is characterized by incommensurate displacive modulations in the Pb sublattice. The higher temperature intermediate IM phase is characterized by rotations of oxygen octahedra, primarily in the form of anti-phase tilts, which are also present in the lower-temperature AFE2 phase. For the same crystal,Sn-119 Mossbauer effect in the temperature range from 300 K to 600 K has been used to study phase transitions mechanism. Two kinds of quadruple splitting have been found. It implies that two different environments of the central Sn ion exist. The observed two kinds of quadruple splitting do not disappear in the whole investigated temperature range which confirm that even far aboveT(C)the structure of paraelectric phase is locally non-centrosymmetric.
Title:
High-Temperature Evolution of the Incommensurate Composite Crystal Ca0.83CuO2
Authors:
Righi, L, Merlini, M, Gemmi, M
Source:
CRYSTALS 10 (7) 10.3390/cryst10070630 JUL 2020
Abstract:
The crystal structure of the composite crystal Ca(0.83)CuO(2)was investigated by synchrotron powder diffraction at high temperature. The incommensurate modulated structure was firstly analyzed at room temperature (RT) and successfully solved by adopting the (3D + 1)-dimensional symmetry P2(1)/m(alpha 0 gamma)0s. The composite crystal is featured by a non-uniform distribution of Ca ions occupying octahedral sites formed by the spatial arrangement by the infinite 1D CuO(2)chains. By approaching 500 K, Ca(0.83)CuO(2)undergoes a structural rearrangement ruled by the shrinking of the Ca interatomic distances. The high-temperature crystalline phase is characterized by a different incommensurate periodicity requiring the recombination of the Ca/CuO(2)balance featuring the composite intergrowth of the two almost independent sub-structures. We ascertain that the new crystalline form is stable up to 950 K near to the limit of the thermal decomposition.
SCIENCE 369 (6504):680 10.1126/science.aay7356 AUG 7 2020
Abstract:
Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMn7O12. In analogy with magnetism, the electric dipole helicoidal texture is stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a nonchiral cubic structure to an incommensurate electric dipole and orbital helix via an intermediate density wave.
Title:
Hierarchical domain structures in (Pb,La)(Zr, Sn, Ti)O-3 antiferroelectric ceramics
CERAM INT 46 (14):22575 10.1016/j.ceramint.2020.06.018 OCT 1 2020
Abstract:
(Pb, La)(Zr, Sn, Ti)O-3 (PLZST) ceramic is one of the most prospective antiferroelectric (AFE) materials for variety of functional applications including energy storage and converter. Systematic structural investigation of domain structures should be of fundamental importance for understanding the structure-property relationship in AFE ceramics. In this study, the hierarchical domain structures and modulated structures correlated to the compositional variation in (Pb0.97La0.02) (Zr0.50SnxTi0.50-x)O-3 (x = 0.375, 0.45 and 0.50) were observed and in- vestigated in details by transmission electron microscopy. The PLZST ceramics show exclusively incommensurate modulated structures (IMS) whose modulation period changed from 9.37 to 6.15 and to 4.04 with increasing of the x value. The hierarchical domain structures include, in decreasing scales, AFE domains, incommensurate domains and nanodomains. The elementary domains in PLZST ceramics are pinstriped nanodomains which were formed based on IMS configuration but by frequent modulation of IMS periodicity and formation of faults. Nanodomains accumulated and then dissociated into incommensurate domains and AFE domains successively. The presently revealed structural characteristics in antiferroelectric PLZST may stimulate future researches on the evolution of IMS-based hierarchical domains under external physical fields, e.g. thermal or electrical, and their correlation to the physical performance.
Title:
A new approach to phason disorder for a decagonal quasicrystal: the moment series expansion of the tiling distribution function for AlCuRh
Authors:
Buganski, I, Strzalka, R, Wolny, J
Source:
J APPL CRYSTALLOGR 53:904 10.1107/S1600576720006251 AUG 2020
Abstract:
A method is proposed of calculating the geometric term of the structure factor for quasicrystals, which enables incorporation of the phason disorder. The scheme is based on the series expansion of the structure factor with moments of the distribution function as coefficients. A distribution function is a mathematical object that is constructed for reference vertices of the tiles in the quasilattice. It encloses the entire structural information of the underlying quasilattice, together with the inherent disorder, necessary to calculate the diffraction pattern. By tuning the value of the distribution moments through the refinement procedure, it is possible to obtain a very good agreement of this new model of the decagonal AlCuRh phase with the experimental data, reflected in the crystallographic R factor of 6.08%. The characteristic bias of the calculated diffraction peak intensities observed for the low-intensity reflections is significantly diminished, confirming its origin being, to some extent, related to phason disorder. Additionally, it is no longer necessary to use the general Debye-Waller factor for phasons, as the new formula accommodates this type of structural disorder. However, the best result was obtained for the model combining the new approach with the Gaussian corrective term.
Title:
Ageing of Al-Mn-Cu-Be Alloys for Stimulating Precipitation of Icosahedral Quasicrystals
Authors:
Boncina, T, Albu, M, Zupanic, F
Source:
METALS-BASEL 10 (7) 10.3390/met10070937 JUL 2020
Abstract:
In this work, the ageing of some Al-Mn-Cu-Be alloys was investigated in the temperature range in which predominantly icosahedral quasicrystalline (IQC) precipitates can form. The alloys were cast into a copper mould, directly aged (T5 heat treatment) between 300 and 440 degrees C for different times. Afterwards, they were examined using scanning and transmission electron microscopy, X-ray diffraction and hardness testing. The main aim of the work was to determine the conditions at which a high number density of spherical icosahedral quasicrystalline precipitates can form. The highest number density of IQC precipitates was obtained at 300 degrees C after prolonged ageing. The spheroidal precipitates had a diameter less than 20 nm. The size of IQC precipitates increased with the increasing temperature, and in addition, decagonal quasicrystalline precipitates appeared. The time to maximum hardness decreased strongly with increasing ageing temperature. The IQC precipitates can form in a fairly broad temperature range in Al-Mn-Cu-Be alloys and that by varying ageing temperature and duration, rather different distributions of precipitates can be obtained. The presence of precipitates caused rather strong aluminium alloys and fast work hardening during initial plastic deformation.
Title:
Three-dimensional exact solution of layered two-dimensional quasicrystal simply supported nanoplates with size-dependent effects
Authors:
Li, Y, Yang, LZ, Zhang, LL, Gao, Y
Source:
APPL MATH MODEL 87:42 10.1016/j.apm.2020.05.001 NOV 2020
Abstract:
A size-dependent plate model is developed to investigate the elastic responses of the multilayered two-dimensional quasicrystal nanoplates based on the nonlocal strain gradient theory for the first time. A nonlocal stress field parameter and a length scale parameter are taken into account in the new model to capture both stiffness-softening and stiffness-hardening size effects. The exact solution for a single-layer two-dimensional quasicrystal simply supported nanoplate is derived by utilizing the pseudo-Stroh formalism in conjunction with the nonlocal strain gradient theory. Afterward, a dual variable and position method is used to deal with the multilayered case. Numerical examples are presented to study the dependence of size-dependent effect on nanoplate length and the influences of scale parameters on the quasicrystal nanoplate subjected to a z-direction mechanical load on its top surface. The proposed model should be useful to verify various nanoplate theories and other numerical methods. (C) 2020 Elsevier Inc. All rights reserved.
Update: 20-Aug-2020
Title:
Mossbauer Study of Rare-earth Ferroborate NdFe3(BO3)(4)
Authors:
Nakamura, S, Masuda, T, Ohgushi, K, Katsufuji, T
Source:
J PHYS SOC JPN 89 (8) 10.7566/JPSJ.89.084703 AUG 15 2020
Abstract:
An Fe-57 Mossbauer spectroscopic study has been conducted on the single-crystal NdFe3(BO3)(4) to investigate its magnetic structure and multiferroicity. In the multiferroic state below the Neel temperature T-N = 29 K, the magnitude of the quadrupole coupling constant e(2)qQ/2 becomes smaller than that in the paramagnetic phase. The decrease is explained by the spin-orbit coupling due to the simultaneous order of Fe3+ and Nd3+ moments, which intensifies the 4f-3d hybridization. This is consistent with the d-p hybridization mechanism as the origin of the multiferroicity. Although the commensurate antiferromagnetic order along the a-axis is confirmed at 25 K, the pronounced incommensurate order is not recognized at 4.2 K. In the latter, it is probable that the Fe3+ moments are distributed only around the a-axis, such as within the +/- 10 degrees region, or that they are fluctuating or frustrating, within the c-plane dynamically with the relaxation time of, for example, 10(-9) s.
Title:
Characterization of reinforced aluminum composites with quasicrystalline particles produced by stir casting/rolling
The continuous requirement of lightweight and high performance materials for the aerospace and automotive industries led to the development of composite materials with a metallic matrix, among which aluminum-based composites. The quasicrystals are metallic materials with a complex atomic structure, which results in a combination of physico-mechanical properties similar to those of some fragile ceramics and intermetallic compounds, such as high strength and hardness, high modulus of elasticity, low coefficient of friction and good resistance to wear and corrosion. These properties are very promising for industrial application, but the fragility of the quasicrystals restricts their application as a reinforcement phase in composites, and can replace the traditionally used ceramic reinforcements such as SiC and Al2O3. In the present work, reinforced aluminum matrix composites with 2,5%, 5% and 10% volume of quasicrystalline particles were produced by the stir cast method of stir casting. Subsequent steps of laminating were performed on all aluminum / quasicrystals composites. The quasicrystalline Al61,7Cu25,5Fe12,3Mn0,5 alloy was obtained by conventional melting and subsequent heat treatment. The mill of the quasicrystalline alloy, to obtain powders, was carried out in a high-energy mill. The microstructure of quasicrystals was investigated by X-ray Diffraction and Scanning Electron Microscopy (SEM). The distribution of the quasicrystalline particles in the aluminum matrix and the influence on the hardness of the composites were investigated by SEM and Vickers Micro-hardness, respectively. The composites presented a good distribution of the quasicrystalline particles in the aluminum matrix, although in some regions the formation of pores and agglomerates occurred. The rolling process promoted a significant increase in hardness in relation to the composites obtained directly from the casting.
Update: 13-Aug-2020
Title:
Frustrated and Allowed Structural Transitions at the Limits of the BaAl4 Type: The (3+2)D Modulated Structure of Dy(Cu(0.18)G(0.82))(3.71)
While elemental substitution is the most common way of tuning properties in solid state compounds, this approach can break down in fantastic ways when the stability range of a structure type is exceeded. In this article, we apply the Frustrated and Allowed Structural Transitions (FAST) principle to understand how structural complexity, in this case incommensurate modulations, can emerge at the composition limits of one common intermetallic framework, the BaAl4 type. While the Dy-Ga binary intermetallic system contains no phases related to the BaAl4 archetype, adding Cu to form a ternary system creates a composition region that is rich in such phases, induding some whose structures remain unknown. We begin with an analysis of electronic and atomic packing issues faced by the hypothetical BaAl4-type phase DyGa4 and a La3Al11-type variant (in which a fraction of Ga t pairs are substituted by single Ga atoms). Through an inspection of its electronic density of states (DOS) distribution and DFT-Chemical Pressure (CP) scheme, we see that the stability of BaAl4-type DyGa4 is limited by an excess of electrons and overly large coordination environments around the Dy atoms, with the latter factor being particularly limiting. The inclusion of Cu into the system is anticipated to soothe both issues through the lowering of the valence electron count and the release of positive CPs between atoms surrounding the Dy atoms. With this picture in mind, we then move to an experimental investigation of the Dy-Cu-Ga system, elucidating the structure of Dy(Cu0.18Ga0.82)(3.71(1)). In this compound, the BaAl4 type is subject to a 2D incommensurate modulation (q(1) = 0.31a* + 0.2b*, q(2) = 0.31a* - 0.2b*), which can be modeled in the (3+2)D superspace group Pmmm(alpha beta 0)000(alpha-beta 0)000. The resulting structure solution contains blocks of the La(3)Al(11 )type, with the corners of these domains serving to shrink the Dy coordination environments. These results highlight how the addition of a well-chosen third element to a binary system with a missing-but plausible-compound (BaAl4-type DyGa4) can bring it to the cusp of stability with intriguing structural consequences.
Title:
Noncoplanar ferrimagnetism and local crystalline-electric-field anisotropy in the quasicrystal approximant Au70Si17Tb13
J PHYS-CONDENS MAT 32 (41) 10.1088/1361-648X/ab997d SEP 30 2020
Abstract:
Neutron scattering experiments have been performed to elucidate magnetic properties of the quasicrystal approximant Au70Si17Tb13, consisting of icosahedral spin clusters in a body-centered-cubic lattice. Bulk magnetic measurements performed on the single crystalline sample unambiguously confirm long-range ordering atT(C)= 11.6 +/- 1 K. In contrast to the simple ferromagnetic response in the bulk measurements, single crystal neutron diffraction confirms a formation of intriguing non-collinear and non-coplanar magnetic order. The magnetic moment direction was found to be nearly tangential to the icosahedral cluster surface in the local mirror plane, which is quite similar to that recently found in the antiferromagnetic quasicrystal approximant Au72Al14Tb14. Inelastic neutron scattering on the powdered sample exhibits a very broad peak centered atPLANCK CONSTANT OVER TWO PI omega similar or equal to 4 meV. The observed inelastic spectrum was explained by the crystalline-electric-field model taking account of the chemical disorder at the fractional Au/Si sites. The resulting averaged anisotropy axis for the crystalline-electric-field ground state is consistent with the ordered moment direction determined in the magnetic structure analysis, confirming that the non-coplanar magnetic order is stabilized by the local uniaxial anisotropy.
Title:
Magnetic properties of icosahedral quasicrystals and their cubic approximants in the Cd-Mg-RE (RE=Gd, Tb, Dy, Ho, Er, and Tm) systems
Authors:
Labib, F, Okuyama, D, Fujita, N, Yamada, T, Ohhashi, S, Sato, TJ, Tsai, AP
Source:
J PHYS-CONDENS MAT 32 (41) 10.1088/1361-648X/ab9343 SEP 30 2020
Abstract:
A systematic investigation has been performed to elucidate effects of rare earth type and structural complexity on magnetic properties of icosahedral quasicrystal (iQC) and their cubic approximants (APs) in the ternary Cd-Mg-RE (RE = Gd, Tb, Dy, Ho, Er, and Tm) systems. At low temperatures, iQCs and 2/1 APs exhibit spin-glass-like freezing for RE = Gd, Tb, Dy, and Ho, while for Er and Tm they do not show freezing behavior down to the base temperature similar to 2 K. The 1/1 APs exhibit either spin-glass-like freezing or antiferromagnetic (AFM) ordering depending on their constituent Mg content. TheT(f)values show increasing trend from iQC to 2/1 and 1/1 APs. In contrast, the absolute values of Weiss temperature for iQCs are larger than those in 2/1 and 1/1 APs, indicating that the total AFM interactions between the neighboring spins are larger in aperiodic, rather than periodic systems. Competing spin interactions originating from the long-range Ruderman-Kittel-Kasuya-Yoshida mechanism along with chemical disorder of Cd/Mg ions presumably account for the observed spin-glass-like behavior in Cd-Mg-RE iQCs and APs.
Title:
Charge density wave and atomic trimerization in layered transition-metal dichalcogenides 1T-MX(2)materials
EPL-EUROPHYS LETT 130 (4) 10.1209/0295-5075/130/47001 MAY 2020
Abstract:
In this work, structural investigation and physical measurements of layered transition-metal dichalcogenides (TMDs) 1T-MX(2)materials revealed a series of remarkable phenomena in correlation with structural transitions. Our results show that notable structural transformations, such as charge density wave (CDW) transitions, atomic ordering, and micro-twinning could be introduced via chemical substitution in 1T-MX2. It was demonstrated that substitution of S by Se in 1T-TaS2-xSex resulted in clear changes in the incommensurability of the CDW state; substitution of Se by Te could destruct the CDW state and yield atomic ordering with visible trimerization of the metallic ions. Furthermore, these structural changes yielded a monoclinic stacking along the c-axis direction. Additionally, on substitution of the metal atoms in 1 -MX2, e.g., 1 -NbTe2, structural distortion in the a -b plane could clearly be observed, and high-density twinning lamella often appeared in the crystals because of enhanced intralayer structural deformation. These phase transitions and their relevant structural features could primarily be corelated with alteration of the superconductivity and other physical properties.
Title:
Effect of Friction on the Degree of Crystallinity of Composite Materials Based on Ultra-high-molecular-weight Polyethylene and Polytetrafluoroethylene with Quasicrystalline Filler Al-Cu-Fe
Authors:
Golovkova, EA, Teplov, AA, Tsetlin, MB, Tikhomirov, SA, Belousov, SI
X-ray diffraction (XRD) analysis of composite samples based on ultra-high-molecular-weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE), with a quasicrystalline filler Al-Cu-Fe, has been carried out. Samples with different filler concentrations were subjected to friction. A change in the degree of crystallinity of the polymer matrix is determined on the basis of the XRD data. Comparison of the diffraction patterns recorded at symmetric scan mode and in the grazing-incidence geometry showed a difference in the structures of the surface layers and sample bulk.
Update: 6-Aug-2020
Title:
Long-range oxygen ordering linked to topotactic oxygen release in Pr(2)NiO(4+delta)fuel cell cathode material
Authors:
Dutta, R, Maity, A, Marsicano, A, Ceretti, M, Chernyshov, D, Bosak, A, Villesuzanne, A, Roth, G, Perversi, G, Paulus, W
Source:
J MATER CHEM A 8 (28):13987 10.1039/d0ta04652c JUL 28 2020
Abstract:
Solid oxide ion conductors are technologically important for oxygen membranes, sensors and solid oxide fuel cells (SOFC). However, oxygen diffusion is a thermally activated process, and materials operating at ambient temperature are rare, as the related diffusion mechanisms are poorly understood. Herein, we report a hidden spontaneous oxygen release reaction that interconverts two stoichiometric phases of the SOFC material Pr(2)NiO(4+delta)with unprecedented structural complexity at ambient temperature. A slight change in the oxygen stoichiometry from delta= 0.25 to delta= 0.225 involves a transition between two competing modulated superstructures, showing long-range translational periodicities up to 94 angstrom. Our findings demonstrate correlated oxygen diffusion within the bulk phase at room temperature, which is accompanied by the formation of long-range modulated superstructures up to the lower mesoscale. Large-scale ordered structures found here for Pr(2)NiO(4+delta)are rather indicative of non-local interactions, and are interpreted to be mediatedviastructural deformations. This unexpected behavior is discussed in terms of an unconventional understanding of low-Toxygen diffusion mechanisms and its potential to conceive and optimize oxygen ion conductors, which are an important class of compounds relevant to technological applications.
Title:
Al-Si ordering in the incommensurately modulated structures of e-plagioclases
Authors:
Jin, SY, Xu, HF, Wang, XP
Source:
ACTA CRYSTALLOGR A 75:A69 10.1107/S0108767319099306 2019
Title:
Successive magnetic transitions and spin structure in the two-leg ladder compound CsFe2Se3 observed by Cs-133 and Se-77 NMR
Authors:
Murase, M, Okada, K, Kobayashi, Y, Hirata, Y, Hashizume, K, Aoyama, T, Ohgushi, K, Itoh, M
Source:
PHYS REV B 102 (1) 10.1103/PhysRevB.102.014433 JUL 20 2020
Abstract:
Cs-133 and Se-77 NMR measurements were conducted on a single crystal to study the local magnetic and electronic properties of the Fe-based compound CsFe2Se3 with a two-leg ladder structure. From the experimental results of the Cs-133 and Se-77 NMR spectra and the Cs-133 nuclear spin-lattice relaxation rate, we found successive magnetic transitions from a paramagnetic phase to an incommensuratelike antiferromagnetic (AFM1) phase with a distributed internal field at T-N1 = 176.5K and then from the AFM1 phase to another commensurate antiferromagnetic (AFM2) phase at T-N2 = 148.5-152.0K. Internal fields of the Cs and Se sites obtained from the angular dependence of the NMR spectra are discussed in terms of the transferred hyperfine tensors governed by site symmetry. For the AFM2 phase, we show that the stripe-type spin structure occurs in two domains, which have magnetic wave vectors of Q = (1/2, 1/2, 0) and (1/2,-1/2, 0). On the contrary, we observed a characteristic modulation of the internal field with magnetic fluctuation between the ladders in the AFM1 phase. To clarify why CsFe2Se3 has a large charge gap although it is a Fe2.5+ mixed valent compound, we investigated the hyperfine coupling constants and found that a hole is doped into the Fe site instead of the Se site. Based on this fact, molecular orbital and double exchange models are proposed for the electronic state.
Title:
Temperature dependent terahertz giant anisotropy and cycloidal spin wave modes in BiFeO(3)single crystal*
Authors:
Liu, F, Jin, ZM, Liu, XM, Fan, YQ, Guo, JJ, Peng, Y, Cheng, ZX, Ma, GH, Zhu, YM
Source:
CHINESE PHYS B 29 (7) 10.1088/1674-1056/ab8dae JUN 2020
Abstract:
THz time-domain spectroscopy (THz-TDS) is used to study the THz-optical properties of a single crystal bismuth ferrite BiFeO3(BFO). It can be found that the anisotropy of BiFeO(3)is strongly dependent on the temperature. A giant birefringence up to around 3.6 is observed at 1 THz. The presence of a spatially modulated cycloidal antiferromagnetic structure leads to spin cycloid resonances (SCR)psi and phi, corresponding to the out-of-plane and in-plane modes of the spin cycloid, respectively. We distinguish the SCR with respect to their response to orthogonal polarizations of the electric fields of the incident THz beam. In addition, we observe a resonance appearing below 140 K, which might be interpreted as an electromagnon mode and related to a spin reorientation transition. Our present observations present that the temperature and polarization, as the external control parameters, can be used to modulate the THz optical properties of BFO single crystal.
Title:
Electronic and Magnetic Properties of BaFeO(3)on the Pt(111) Surface in a Quasicrystalline Approximant Structure
Authors:
Adeagbo, WA, Maznichenko, IV, Ben Hamed, H, Mertig, I, Ernst, A, Hergert, W
Source:
PHYS STATUS SOLIDI B 257 (7) 10.1002/pssb.201900649 JUL 2020
Abstract:
Perovskite-like ABO(3)oxides A = (Ca, Sr, Ba) and B = (Ti, Mn, Fe, Co, Ni) show a large variety of structures and physical properties. Among them is BaTiO3(BTO), one of the most investigated and used perovskites. In a BTO film on Pt(111), the first oxide quasicrystal was discovered. Herein, by means of first-principle methods, the cubic and hexagonal phases of bulk BaFeO3(BFO) are investigated. Both phases show ferromagnetic order. Monolayers and double layers of BFO are studied on a Pt(111) surface. The double-layer configuration of the cubic and hexagonal phases is structurally inequivalent but both double-layer films show antiferromagnetic order. In analogy to the BTO quasicrystal approximant structure on Pt(111), a corresponding BFO structure is investigated. The Fe atoms are surrounded by three oxygen atoms and the resulting FeO(3)units are separated by barium atoms with the total stoichiometry Ba5Fe4O12.
Title:
Phase Transformation of High Velocity Air Fuel (HVAF)-Sprayed Al-Cu-Fe-Si Quasicrystalline Coating
Authors:
Cai, MW, Shen, J
Source:
METALS-BASEL 10 (6) 10.3390/met10060834 JUN 2020
Abstract:
Al-Cu-Fe-Si quasicrystalline coatings were prepared by high velocity air fuel spraying to study their phase transformation during the process. The feedstock powder and coating were phase characterized by scanning electron microscopy, X-ray diffractometry, differential scanning calorimetry, and transmission electron microscopy. Results show that Al(3)Cu(2)phase, a small amount of lambda-Al(13)Fe(4)phase, quasicrystalline phase (QC), amorphous phase, and beta-Al (Cu, Fe, Si) phase were present in the sprayed Al(50)Cu(20)Fe(15)Si(15)powder. For a typical flattened powder particle, the splat periphery was surrounded by a 1 mu m thick amorphous phase. The inside area of the splat was composed of the QC covered by the Al(3)Cu(2)and Si-rich beta-Al (Cu, Fe, Si) phases. Another kind of Cu- rich beta-Al (Cu, Fe, Si) phase can be found close to the amorphous area with a similar composition to the original beta-Al (Cu, Fe, Si) phase in the powder. Different phases were observed when the periphery and inside area of the splat were compared. This result was caused by the difference in the heating and cooling rates.
Title:
Study of thermal and structural characteristics of mechanically milled nanostructured Al-Cu-Fe quasicrystals
MATER CHEM PHYS 251 10.1016/j.matchemphys.2020.123071 SEP 1 2020
Abstract:
Single phase nanostructured Al-Cu-Fe quascrystalline powders, with various grain sizes, were synthesized by low intensity mechanical milling of spray deposited icosahedral quasicrystals. Structural evolution as a function of temperature was studied in-situ using high energy synchrotron X-ray diffraction. Coefficient of thermal expansion was calculated using the change in d-spacing. The diffraction results revealed a change in the lattice parameter, grain size and quasi-lattice strain with increasing temperature. During heating, onset of appreciable grain growth in different nanostructured quascrystalline powders was observed at a certain critical temperature, which was closely related with the milling time. Quasicrystalline powders milled for 1 and 80 h exhibited appreciable grain coarsening at 923 and 573 K, respectively. For all the samples, the activation energy for grain growth (E-gg) was similar to 30-35 kJ/mol. In addition, the beta Al(Cu,Fe) phase with B2-type structure evolved during heating of powder samples milled for 10 h. Onset temperature for beta phase formation showed an inverse relationship with the milling time.
Title:
Influence of Ag Addition on Thermal Stability and Thermophysical Properties of Ti-Zr-Ni Quasicrystals
We study the effects of Ag addition on thermal stability and thermophysical properties of Ti-Zr-Ni icosahedral quasicrystals. The Ag addition results in increasing the coherence length and thermal stability of the icosahedral phase (i-phase) of the as-cast Ti(35.2)Zr(43.8)Ni(21)alloy, which are maximized at around 4 at.% Ag addition. Differential scanning calorimetry (DSC) and electrostatic levitation (ESL) experiments reveal that the addition suppresses thei-phase decomposition on heating and cooling. We find that considerable amount of thei-phase remains in the samples processed by radiational cooling in ESL as the Ag concentration increases. These results demonstrate that Ag addition stabilizes thei-phase of the Ti(35.2)Zr(43.8)Ni(21)alloy. No anomalous effect of Ag addition is found on density and viscosity of the Ti(35.2)Zr(43.8)Ni(21)liquid.
Title:
Extension of Elastic Models to Decagonal Quasicrystals
Authors:
Li, W, Shi, YQ
Source:
CRYSTALS 10 (6) 10.3390/cryst10060469 JUN 2020
Abstract:
The main design of this paper is to adopt potential functions for solving plane defect problems originating from two-dimensional decagonal quasicrystals. First, we analyze the strict potential function theory for the plane problems of two-dimensional quasicrystals. To clarify effectiveness of the method, we give some examples and the results which can be precisely determined, including the elasticity and fracture theories of two-dimensional quasicrystals. These results maybe play a positive role in studying the fracture of two-dimensional quasicrystals in the future.
Update: 30-Jul-2020
Title:
Structural distortion and incommensurate noncollinear magnetism in EuAg4As2
Authors:
Shen, B, Hu, CW, Cao, HB, Gui, X, Emmanouilidou, E, Xie, WW, Ni, N
Source:
PHYS REV MATER 4 (6) 10.1103/PhysRevMaterials.4.064419 JUN 25 2020
Abstract:
Layered pnictide materials have provided a fruitful platform to study various emergent phenomena, including superconductivity, magnetism, charge density waves, etc. Here we report the observation of structural distortion and noncollinear magnetism in layered pnictide EuAg4As2 via transport, magnetization, single crystal x-ray, and neutron diffraction data. EuAg4As2 single crystal shows a structural distortion at 120 K, where two sets of superlattice peaks with the propagation vectors of q(1) = +/-(0, 0.25, 0.5) and q(2) = +/-(0.25, 0, 1) emerge. Between 9-15 K, the hexagonal Eu2+ sublattice enters an unpinned incommensurate magnetic state, with magnetic Bragg reflections pictured as circular sectors. Below 9 K, it orders in an incommensurate noncollinear antiferromagnetic state with a well-defined propagation wavevector of (0, 0.1, 0.12) and a very rare magnetic structure, which is helical along the c axis and cycloidal along the b axis with a moment of 6.4 mu(B)/Eu2+. Furthermore, rich magnetic phases under magnetic fields, large magnetoresistance, and strong coupling between charge carriers and magnetism in EuAg4As2 are revealed.
Title:
A new modulated crystal structure of the ANS complex of the St John's wort Hyp-1 protein with 36 protein molecules in the asymmetric unit of the supercell
Authors:
Smietanska, J, Sliwiak, J, Gilski, M, Dauter, Z, Strzalka, R, Wolny, J, Jaskolski, M
Source:
ACTA CRYSTALLOGR D 76:653 10.1107/S2059798320006841 JUL 1 2020
Abstract:
Superstructure modulation, with violation of the strict short-range periodic order of consecutive crystal unit cells, is well known in small-molecule crystallography but is rarely reported for macromolecular crystals. To date, one modulated macromolecular crystal structure has been successfully determined and refined for a pathogenesis-related class 10 protein fromHypericum perforatum(Hyp-1) crystallized as a complex with 8-anilinonaphthalene-1-sulfonate (ANS) [Sliwiaket al.(2015),Acta Cryst.D71, 829-843]. The commensurate modulation in that case was interpreted in a supercell with sevenfold expansion alongc. When crystallized in the additional presence of melatonin, the Hyp-1-ANS complex formed crystals with a different pattern of structure modulation, in which the supercell shows a ninefold expansion ofc, manifested in the diffraction pattern by a wave of reflection-intensity modulation with crests atl= 9nandl= 9n +/- 4. Despite complicated tetartohedral twinning, the structure has been successfully determined and refined to 2.3 angstrom resolution using a description in a ninefold-expanded supercell, with 36 independent Hyp-1 chains and 156 ANS ligands populating the three internal (95 ligands) and five interstitial (61 ligands) binding sites. The commensurate superstructures and ligand-binding sites of the two crystal structures are compared, with a discussion of the effect of melatonin on the co-crystallization process.
Title:
Exciton and exciton-magnon photoluminescence in the antiferromagnet CuB2O4
PHYS REV B 102 (3) 10.1103/PhysRevB.102.035128 JUL 16 2020
Abstract:
Copper metaborate CuB2O4 crystallizes in a unique noncentrosymmetric structure, becomes antiferromagnetically ordered below T-N1 = 20 K, and exhibits a great diversity in magnetic, optical, and magneto-optical properties. In particular, it shows strong photoluminescence rarely observed before in other magnetically ordered copper oxides in which magnetic properties are defined by magnetic Cu2+ (3d(9), S = 1/2) ions. Here we report on the detailed spectroscopic study of the photoluminescence originating from the Cu2+ ions. Our investigations are focused on understanding the energy-level scheme of the multiple excitations below the energetically lowest, crystal-field-split d-d electronic transition at 1.405 eV. We identify multiple emission lines, and among them we distinguish three sets of lines, each composed of an exciton line and a satellite attributed to magnon-assisted exciton recombination. The emission intensity of the three sets changes strongly in the temperature range 1.7-40 K, showing pronounced correlations with the magnetic phase transitions between the commensurate and incommensurate phases. Photoluminescence excitation spectra and time-resolved emission dynamics give closer insight into the energy relaxation channels populating the exciton-magnon sets.
Title:
Atomic-Scale Observation of Topological Vortices in the Incommensurate Charge Density Wave of 2H-TaSe2
Authors:
Lim, S, Kim, J, Won, C, Cheong, SW
Source:
NANO LETT 20 (7):4801 10.1021/acs.nanolett.0c00539 JUL 8 2020
Abstract:
It has been only recently realized that topological vortices associated with structural distortions or ordered spins are rather common in numerous materials where long-range interactions are not dominant. Incommensurate modulations that frequently occur in charge density wave (CDW) materials are often understood in terms of discommensurations with a periodic phase shift. The accumulation of a one-dimensional (1D) phase shift can result in, for example, CDW dislocations in 2H-TaSe2 with incommensurate CDW (I-CDW). Since any atomic-scale experimental investigation of CDW dislocations in 2H-TaSe2 has been lacking, we have performed the atomic-scale observation of 2H-TaSe2 with I-CDW, stabilized with Pd intercalation or strain, with scanning probe microscopy, and unveiled the existence of topological Z(6) or Z(4) vortices with topologically protected 2D winding movements of atomic displacement vectors. The discovery opens the ubiquitous nature of topological vortex domains and a new avenue to explore new facets of various incommensurate modulations or discommensurations.
Title:
Order-parameter coupling and strain relaxation behavior of Ti50Pd50-xCrx martensites
PHYS REV B 102 (1) 10.1103/PhysRevB.102.014105 JUL 16 2020
Abstract:
A group theoretical model is proposed for linear/quadratic coupling between order parameters which arise from electronic and soft-mode instabilities in doped shape memory alloys, together with coupling to symmetry breaking shear strains. This model is tested by using resonant ultrasound spectroscopy (RUS) to follow the elastic and anelastic anomalies which accompany transitions from B2 to B19, 9R, and incommensurate structures in Ti50Pd50-xCrx alloy samples (0 <= x <= 12). The pure soft-mode transition gives rise to an incommensurate structure but without any associated changes in the shear modulus, implying that coupling with shear strains is weak. By way of contrast, the observed pattern of softening ahead of and stiffening below the martensitic transition is typical of pseudoproper ferroelastic behavior and confirms that there is strong bilinear coupling of the tetragonal shear strain to the order parameter associated with irrep Gamma(+)(3) of the parent space group. The second order parameter has the symmetry properties of M-5(-) in TiPd or of a point along the Sigma line of the Brillouin zone for the 9R and incommensurate structures with high Cr contents. Comparison of shear modulus data for Cr-rich samples obtained by RUS at 10(5) - 10(6) Hz with previously reported Young's modulus data obtained for different samples by dynamical mechanical analysis at similar to 0.1-10 Hz has not revealed the dispersion with frequency that would be expected for a glass transition governed by Vogel-Fulcher dynamics. The two techniques differ in the magnitude of effective applied stress, however, and differences in chemical homogeneity between samples or decomposition during high-temperature measurements might also be a factor.
Title:
Atomic-Scale Tuning of Tsai-Type Clusters in RE-Au-Si Systems (RE = Gd, Tb, Ho)
Tsai-type quasicrystals and approximants are distinguished by a cluster unit made up of four concentric polyhedral shells that surround a tetrahedron at the center. Here we show that for Tsai-type 1/1 approximants in the RE-Au-Si systems (RE = Gd, Tb, Ho) the central tetrahedron of the Tsai clusters can be systematically replaced by a single RE atom. The modified cluster is herein termed a "pseudo-Tsai cluster" and represents, in contrast to the conventional Tsai cluster, a structural motif without internal symmetry breaking. For each system, single-phase samples of both pseudo-Tsai and Tsai-type 1/1 approximants were independently prepared as millimeter-sized, faceted, single crystals using the self-flux synthesis method. The full replacement of tetrahedral moieties by RE atoms in the pseudo-Tsai 1/1 approximants was ascertained by a combination of single-crystal and powder diffraction studies, as well as energy dispersive X-ray spectroscopy (EDX) analyses with a scanning electron microscope (SEM). Differential scanning calorimetry (DSC) studies revealed distinctly higher decomposition temperatures, by 5-35 K, for the pseudo-Tsai phases. Furthermore, the magnetic properties of pseudo-Tsai phases are profoundly and consistently different from the Tsai counterparts. The onset temperatures of magnetic ordering (T-mag) are lowered in the pseudo-Tsai phases by similar to 30% from 24 to 17 K, 11.5 to 8 K, and 5 to 3.5 K in the Gd-Au-Si, Tb-Au-Si, and Ho-Au-Si systems, respectively. In addition, the Tb-Au-Si and Ho-Au-Si systems exhibit some qualitative changes in their magnetic ordering, indicating decisive changes in the magnetic state/structure by a moment-bearing atom at the cluster center.
Title:
Crystal structure, electronic structure and thermoelectric properties of beta- and gamma-Zn4Sb3 thermoelectrics: a (3+1)-dimensional superspace group approach
Authors:
Yoshioka, S, Hayashi, K, Yokoyama, A, Saito, W, Li, HZ, Takamatsu, T, Miyazaki, Y
Source:
J MATER CHEM C 8 (27):9205 10.1039/d0tc01497d JUL 21 2020
Abstract:
Thermoelectric (TE) materiaLs are promising candidates for solving today's energy problem owing to their ability to directly convert waste heat into electricity via the Seebeck effect. One of the most efficient TE materiaLs known is Zn4Sb3. To understand its high efficiency, a novel composite crystaL structure model for 13- and 7 -phases of Zn4Sb3 is constructed using a (3 + 1) -dimensional ((3 + 1)-D) superspace group approach. This (3 + 1)-D model is expressed as [Zn3+,5S19][Sb], with the superspace group of R3m(00)))0s. The [Zn3+,5S19] and [Sb] subsystems have same a- and b -axis Lengths but a different c -axis Length. The (3 + 1)-D model contains four atomic sites: a Zn(1) normal site, an interstitial Zn (Zn;) site and an Sb(1) site in the [Zn3+,5S19] subsystem and an Sb(2) site in the [Sb] subsystem, which is different from a conventional 3D model containing additional Zn; sites. The crystaL structures of 13- and 7-Zn4Sb3 are investigated via powder and synchrotron X-ray diffraction (XRD) measurements. The XRD patterns are weLL analysed by the (3 + 1)-D modeL. The occupancies of Zn(1), Sb(1) and Sb(2) sites are 100%, whereas the Zn; occupancy changes depending on the heating time during the preparation of 13-Zn4Sb3. Moreover, electronic density of states (DOS) of 13-Zn4Sb3 with and without Zn; is caLcuLated based on the (3 + 1)-D modeL, demonstrating a dose relationship between the DOS and the Zn; occupancy. The caLcuLated TE properties, such as Seebeck coefficient, electrical conductivity and power factor, also depend on the Zn; occupancy.
Update: 23-Jul-2020
Title:
Charge density wave with anomalous temperature dependence in UPt2Si2
Authors:
Lee, J, Prokes, K, Park, S, Zaliznyak, I, Dissanayake, S, Matsuda, M, Frontzek, M, Stoupin, S, Chappell, GL, Baumbach, RE, Park, C, Mydosh, JA, Granroth, GE, Ruff, JPC
Source:
PHYS REV B 102 (4) 10.1103/PhysRevB.102.041112 JUN 9 2020
Abstract:
Using single-crystal neutron and x-ray diffraction, we discovered a charge density wave (CDW) below 320 K, which accounts for the long-sought origin of the heat capacity and resistivity anomalies in UPt2Si2. The modulation wave vector, Q(mod), is intriguingly similar to the incommensurate wave vector of URu2Si2. Q(mod) shows an unusual temperature dependence, shifting from commensurate to incommensurate position upon cooling and becoming locked at aproximately (0.42 0 0) near 180 K. Bulk measurements indicate a crossover toward a correlated coherent state around the same temperature, suggesting an interplay between the CDW and Kondo-lattice-like coherence before coexisting antiferromagnetic order sets in at T-N = 35 K.
Title:
Quaternary Ln(x)La((1-x))S-TaS2 nanotubes (Ln=Pr, Sm, Ho, and Yb) as a vehicle for improving the yield of misfit nanotubes
Authors:
Serra, M, Lajaunie, L, Sreedhara, MB, Miroshnikov, Y, Pinkas, I, Calvino, JJ, Enyashin, AN, Tenne, R
Source:
APPL MATER TODAY 19 10.1016/j.apmt.2020.100581 JUN 2020
Abstract:
The non-stoichiometric misfit layered compounds (MLC) of the general formula ((MX)(1+y))(m)(TX2)(n) (abbreviated herein as MX-TX2) have been investigated quite extensively over the last 30 years. Here MX is a atomic slab of a material with distorted rocksalt structure and TX2 is a layered compound with hexagonal (octahedral) coordination between the metal T atom and the chalcogen X atom. Recognizing the mismatch between the two (MX and TX2) sublattices, nanotubes from the MLC of different compositions were described in the past. In particular, semimetallic nanotubes belonging to the family LnX-TaX2 with Ln = rare earth atom and X = S, Se, Te have been studied in the past. While some of them, like LaS-TaS2 were obtained with moderately high yields, others like YbS-TaS2 were scarce. In the present study, a new strategy for promoting the yield of such MLC nanotubes by alloying the LaS sublattice with another Ln atom is proposed. Detailed transmission electron microscopy investigation of the (mixed) Ln(x)La((1-x))S-TaS2 (Ln = Pr, Sm, Ho, Yb) nanotubes show clearly that the substituting Ln atom resides in the rocksalt LaS sublattice of the nanotubes. Raman measurements show distinct differences between mixed tubes with open-shell (Pr, Sm, Ho) and close-shell (La, Yb) rare-earth atoms. Density functional calculations show that the interplay between two important factors determine the enhanced stability of the mixed nanotubes- the size and electronic structure of the substituting rare-earth atom. The smaller is the substituting rare-earth atom (larger Z number), the more dissimilar it is to the original La atom. This dissimilarity enhances the incommensurability between the Ln(x)La((1-x))5 and the TS2 subunits, promoting thereby the stability of the mixed MLC. However, the electronic structure of the Ln atom was found to play a more significant role. The MLC lattice of the LaS-TaS2 is electron-rich and consequently the 4d(z)(2) level of Ta is full. The unoccupied 4f levels of the substituent open-shell atoms (Pr, Sm, Ho), which are positioned below the Fermi level, serve as electron acceptors. Consequently, the Ln substitution is found to enhance the stability of the mixed lattice and nanotubes thereof. This strategy can be employed for enhancing the yield of these and other misfit nanotubes using different substituents of the right size and energy profile. (C) 2020 Published by Elsevier Ltd.
MATER CHARACT 166 10.1016/j.matchar.2020.110424 AUG 2020
Abstract:
We report a thermodynamically stable quaternary Al59Cr23Fe8Si10 decagonal quasicrystal (DQC), whose stability was verified by annealing at both high and low temperature for 168 or 360 h. The atomic scale structural features of the quasiperiodic plane were revealed based on high-angle annular dark-field scanning transmission electron microscopy images and the reasons for the stability of the Al59Cr23Fe8Si10 DQC are briefly discussed based on the structural characteristics. We argue that the 2 nm decagonal clusters with broken tenfold symmetry and their random filing in the quasiperiodic plane of the Al59Cr23Fe8Si10 DQC enhance the stability.
Update: 17-Jul-2020
Title:
Magnetic phase transitions and spin density distribution in the molecular multiferroic system GaV4S8
Authors:
Dally, RL, Ratcliff, WD, Zhang, LY, Kim, HS, Bleuel, M, Kim, JW, Haule, K, Vanderbilt, D, Cheong, SW, Lynn, JW
Source:
PHYS REV B 102 (1) 10.1103/PhysRevB.102.014410 JUL 7 2020
Abstract:
We have carried out neutron diffraction and small-angle neutron scattering measurements on a high-quality single crystal of the cubic lacunar spinel multiferroic, GaV4S8, as a function of magnetic field and temperature to determine the magnetic properties for the single electron that is located on the tetrahedrally coordinated V-4 molecular unit. Our results are in good agreement with the structural transition at 44 K from cubic to rhombohedral symmetry where the system becomes a robust ferroelectric, while long-range magnetic order develops below 13 K in the form of an incommensurate cycloidal magnetic structure, which can transform into a Neel-type skyrmion phase in a modest applied magnetic field. Below 5.9(3) K, the crystal enters a ferromagnetic phase, and we find the magnetic order parameter indicates a long-range-ordered ground state with an ordered moment of 0.23(1) mu(B) per V ion. Both polarized and unpolarized neutron data in the ferroelectric-paramagnetic phase have been measured to determine the magnetic form factor. The data are consistent with a model of the single spin being uniformly distributed across the V-4 molecular unit, rather than residing on the single apical V ion, in substantial agreement with the results of first-principles theory. In the magnetically ordered state, polarized neutron measurements are important since both the cycloidal and ferromagnetic order parameters are clearly coupled to the ferroelectricity, causing the structural peaks to be temperature and field dependent. For the ferromagnetic ground state, the spins are locked along the [1,1,1] direction by a surprisingly large anisotropy.
Title:
The effect of film thickness on the modulated spin structure of terbium manganite
Authors:
Chupis, IE
Source:
LOW TEMP PHYS+ 46 (6):602 10.1063/10.0001243 JUN 2020
Abstract:
The Ginzburg-Landau method is used to conduct a phenomenological analysis of how the sample thickness impacts the incommensurate spin states in the TbMnO(3)centrosymmetric ferroelectromagnet. The analysis is performed using the first-harmonic approximation, and provides a qualitative confirmation of the experimentally observed decrease in the film modulation vector at a decreasing temperature, as well as an increase in the vector as the film thickness decreases at a constant temperature. The effect that the film thickness has on the first- and second-order phase transition lines is analyzed, and the tendency of the incommensurate spin phase volume to decrease with a decrease in the film thickness is demonstrated.
Title:
Momentum-Dependent Magnon Lifetime in the Metallic Noncollinear Triangular Antiferromagnet CrB2
Authors:
Park, P, Park, K, Kim, T, Kousaka, Y, Lee, KH, Perring, TG, Jeong, J, Stuhr, U, Akimitsu, J, Kenzelmann, M, Park, JG
Source:
PHYS REV LETT 125 (2) 10.1103/PhysRevLett.125.027202 JUL 7 2020
Abstract:
Noncollinear magnetic order arises for various reasons in several magnetic systems and exhibits interesting spin dynamics. Despite its ubiquitous presence, little is known of how magnons, otherwise stable quasiparticles, decay in these systems, particularly in metallic magnets. Using inelastic neutron scattering, we examine the magnetic excitation spectra in a metallic noncollinear antiferromagnet CrB2, in which Cr atoms form a triangular lattice and display incommensurate magnetic order. Our data show intrinsic magnon damping and continuumlike excitations that cannot be explained by linear spin wave theory. The intrinsic magnon linewidth Gamma(q, E-q) shows very unusual momentum dependence, which our analysis shows to originate from the combination of two-magnon decay and the Stoner continuum. By comparing the theoretical predictions with the experiments, we identify where in the momentum and energy space one of the two factors becomes more dominant. Our work constitutes a rare comprehensive study of the spin dynamics in metallic noncollinear antiferromagnets. It reveals, for the first time, definite experimental evidence of the higher-order effects in metallic antiferromagnets.
Title:
Impact of structural complexity and disorder on lattice dynamics and thermal conductivity in the o-Al13Co4 phase
Authors:
Lory, PF, Giordano, VM, Gille, P, Euchner, H, Mihalkovic, M, Pellegrini, E, Gonzalez, M, Regnault, LP, Bastie, P, Schober, H, Pailhes, S, Johnson, MR, Grin, Y, de Boissieu, M
Source:
PHYS REV B 102 (2) 10.1103/PhysRevB.102.024303 JUL 7 2020
Abstract:
Combining inelastic neutron and x-ray scattering with atomic scale simulation, we report on a comprehensive study of the lattice dynamics and its relationship with the low thermal conductivity of the o-Al13Co4 phase, a periodic approximant (about 100 atoms per cell) to a decagonal quasicrystal. The obtained experimental data, phonon lifetimes, and temperature-independent lattice thermal conductivity can only be fully described by molecular dynamics simulations, using oscillating pair potential, when the disorder of 12 Al sites is properly taken into account. Our results pave the way for a detailed understanding of materials where structural complexity and local disorder are at play.
Title:
Dodecagonal Quasicrystals in Mesoporous Silica: A New Route from Hard- to Soft-Sphere Packings
Authors:
Wang, YY, Deng, QZ, Fujita, N, Han, L
Source:
CHEM MATER 32 (12):5236 10.1021/acs.chemmater.0c01382 JUN 23 2020
Abstract:
Quasicrystals attract considerable attention due to their unique physical and structural properties. An increasing number of quasicrystals have been reported from self-assembly processes in soft matter systems; however, no generic synthesis conditions that yield mesoscopic quasicrystals have been fully clarified yet. This paper presents a new synthesis system for mesoporous silica crystals that yields dodecagonal quasicrystals, employing a gemini cationic surfactant as a template, carboxyethylsilanetriol sodium salt as a costructure-directing agent, and tetraethyl orthosilicate as a silica source. We show structural changes from cubic close packing to a composite of cubic and hexagonal close packings, and further to tetrahedral close packings derived from dodecagonal tilings with squares and triangles as the concentration of the co-structure directing agent is gradually decreased. These structural changes are attributable to the softening of the spherical micelles, i.e., the building units of the structure while in solution, considering that area-minimizing duals are energetically favored by the packing of soft spheres. A possible relevance of the present findings to a recent prediction on the stability of dodecagonal quasicrystalline dry foams is pointed out.
Title:
Insight into perovskite antiferroelectric phases: Landau theory and phase field study
Authors:
Liu, Z, Xu, BX
Source:
SCRIPTA MATER 186:136 10.1016/j.scriptamat.2020.04.040 SEP 2020
Abstract:
The appearance of commensurate and incommensurate modulations in perovskite antiferroelectrics (AFEs) is not understood. And the elastic property of the AFE domain boundaries is lack of investigation. In this work, a novel Landau theory is proposed to understand the transformation of AFE commensurate and incommensurate phases, by considering the coupling between the oxygen octahedral tilt mode and the polar mode. The relationship between the modulation periodicity and temperature is obtained. Using the phase field study, we show that the suppressed polarization at the AFE domain boundaries can lead to a remanent polarization and local elastic stress field. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Title:
An atomic scale study of two-dimensional quasicrystal nucleation controlled by multiple length scale interactions
Authors:
Tang, S, Wang, ZJ, Wang, JC, Jiang, K, Liang, CP, Ma, YZ, Liu, WS, Du, Y
Source:
SOFT MATTER 16 (24):5718 10.1039/c9sm02243k JUN 28 2020
Abstract:
Formation of quasicrystal structures has always been mysterious since the discovery of these magic structures. In this work, the nucleation of decagonal, dodecagonal, heptagonal, and octagonal quasicrystal structures controlled by the coupling among multiple length scales is investigated using a dynamic phase-field crystal model. We observe that the nucleation of quasicrystals proceeds through local rearrangement of length scales,i.e., the generation, merging and stacking of 3-atom building blocks constructed by the length scales, and accordingly, propose a geometric model to describe the cooperation of length scales during structural transformation in quasicrystal nucleation. Essentially, such cooperation is crucial to quasicrystal formation, and controlled by the match and balance between length scales. These findings clarify the scenario and microscopic mechanism of the structural evolution during quasicrystal nucleation, and help us to understand the common rule for the formation of periodic crystal and quasicrystal structures with various symmetries.
Title:
Kohn-Luttinger Mechanism Driven Exotic Topological Superconductivity on the Penrose Lattice
Authors:
Cao, Y, Zhang, YY, Liu, YB, Liu, CC, Chen, WQ, Yang, F
Source:
PHYS REV LETT 125 (1) 10.1103/PhysRevLett.125.017002 JUN 30 2020
Abstract:
The Kohn-Luttinger mechanism for unconventional superconductivity (SC) driven by weak repulsive electron-electron interactions on a periodic lattice is generalized to the quasicrystal (QC) via a real-space perturbative approach. The repulsive Hubbard model on the Penrose lattice is studied as an example, on which a classification of the pairing symmetries is performed and a pairing phase diagram is obtained. Two remarkable properties of these pairing states are revealed, due to the combination of the presence of the point-group symmetry and the lack of translation symmetry on this lattice. First, the spin and spacial angular momenta of a Cooper pair is decorrelated: for each pairing symmetry, both spin-singlet and spintriplet pairings are possible even in the weak-pairing limit. Second, the pairing states belonging to the 2D irreducible representations of the D5 point group can be time-reversal-symmetry-breaking topological SCs carrying spontaneous bulk super current and spontaneous vortices. These two remarkable properties are general for the SCs on all QCs, and are rare on periodic lattices. Our work starts the new area of unconventional SCs driven by repulsive interactions on the QC.
Title:
Extraction of Two-Dimensional Aluminum Alloys from Decagonal Quasicrystals
ACS NANO 14 (6):7435 10.1021/acsnano.0c03081 JUN 23 2020
Abstract:
Atomically thin metallic alloys are receiving increased attention due to their prospective applications as interconnects/contacts in two-dimensional (2D) circuits, sensors, and catalysts, among others. In this work, we demonstrate an easily scalable technique for the synthesis of 2D metallic alloys from their 3D quasicrystalline precursors. We have used aluminum (Al)-based single-phase decagonal quasicrystal Al66Co17Cu17 alloy to extract the corresponding 2D alloy structure. The 2D layered Al alloy possesses 2-fold decagonal quasicrystalline symmetry and consists of two- or three-layer-thick sheets with a lateral dimension of microns. These 2D metallic layers were combined with the atomic layers of tungsten disulfide to form the stacked heterostructures, which is demonstrated to be a stable and efficient catalyst for hydrogen evolution reaction.
Title:
Dendrite Morphology Evolution of Al6Mn Phase in Suction Casting Al-Mn Alloys
Authors:
Chen, ZW, Hou, Y, Xie, B, Zhang, Q
Source:
MATERIALS 13 (10) 10.3390/ma13102388 MAY 2020
Abstract:
The effects of solute element content and cooling rate on the morphology of Al6Mn phase in suction casting Al-Mn alloys were investigated by transmission electron microscope, scanning electron microscope, and X-ray diffractometer. Results show that Al6Mn dendrite morphology with different degrees of development can occur in the microstructure of as-cast Al-Mn alloys. For the Al-4 wt.% Mn alloy, there are small amounts of block Al6Mn crystals at the center of sample, while we see a block Al6Mn phase and a feathery Al6Mn phase in the sample of Al-6 wt.% Mn alloy. Moreover, the block Al6Mn phases in the Al-8 wt.% Mn alloy disappear, and only snowflake-like Al6Mn phases play a dominant role in the microstructure. However, with an increase in Mn content to 10 wt.%, more dendritic trunks are formed, and secondary dendrite arms are degraded more seriously due to the formation of an icosahedral quasicrystal in suction casting. In addition to the effect of Mn content on Al6Mn morphology, with the increase in cooling rate from the center to the edge of samples, the outline diameter of equiaxed dendrite decreases. The evolution of Al6Mn dendrite morphology and the formation of quasicrystal are further discussed.
Update: 9-Jul-2020
Title:
Unprecedented generation of 3D heterostructures by mechanochemical disassembly and re-ordering of incommensurate metal chalcogenides
NAT COMMUN 11 (1) 10.1038/s41467-020-16672-0 JUN 12 2020
Abstract:
Three-dimensional heterostructures are usually created either by assembling two-dimensional building blocks into hierarchical architectures or using stepwise chemical processes that sequentially deposit individual monolayers. Both approaches suffer from a number of issues, including lack of suitable precursors, limited reproducibility, and poor scalability of the preparation protocols. Therefore, development of alternative methods that enable preparation of heterostructured materials is desired. We create heterostructures with incommensurate arrangements of well-defined building blocks using a synthetic approach that comprises mechanical disassembly and simultaneous reordering of layered transition-metal dichalcogenides, MX2, and non-layered monochalcogenides, REX, where M=Ta, Nb, RE=Sm, La, and X=S, Se. We show that the discovered solid-state processes are rooted in stochastic mechanochemical transformations directed by electronic interaction between chemically and structurally dissimilar solids toward atomic-scale ordering, and offer an alternative to conventional heterostructuring. Details of composition-structure-properties relationships in the studied materials are also highlighted. 3D heterostructures offer properties that are inaccessible in bulk single-phase solids, but synthetic approaches are limited. The authors use mechanochemical reshuffling of binary precursors and subsequent annealing to design structurally aligned misfit heterostructures with well-defined atomic arrangements.
Title:
Imaging the energy gap modulations of the cuprate pair-density-wave state
Authors:
Du, ZY, Li, H, Joo, SH, Donoway, EP, Lee, J, Davis, JCS, Gu, G, Johnson, PD, Fujita, K
The defining characteristic(1,2) of Cooper pairs with finite centre-of-mass momentum is a spatially modulating superconducting energy gap Delta(r), where r is a position. Recently, this concept has been generalized to the pair-density-wave (PDW) state predicted to exist in copper oxides (cuprates)(3,4). Although the signature of a cuprate PDW has been detected in Cooper-pair tunnelling(5), the distinctive signature in singleelectron tunnelling of a periodic Delta(r) modulation has not been observed. Here, using a spectroscopic technique based on scanning tunnelling microscopy, we find strong Delta(r) modulations in the canonical cuprate Bi2Sr2CaCu2O8+delta that have eight-unit-cell periodicity or wavevectors Q approximate to (2 pi/a(0))(1/8, 0) and Q approximate to (2 pi/a(0))(0, 1/8) (where a(0) is the distance between neighbouring Cu atoms). Simultaneous imaging of the local density of states N(r, E) (where E is the energy) reveals electronic modulations with wavevectors Q and 2Q, as anticipated when the PDW coexists with superconductivity. Finally, by visualizing the topological defects in these N(r, E) density waves at 2Q, we find them to be concentrated in areas where the PDW spatial phase changes by pi, as predicted by the theory of half-vortices in a PDW state(6,7). Overall, this is a compelling demonstration, from multiple single-electron signatures, of a PDW state coexisting with superconductivity in Bi2Sr2CaCu2O8+delta.
Title:
Microstructure/mechanical behavior relationships in upset-forged powder-processed Al alloys containing icosahedral quasicrystalline dispersoids
MAT SCI ENG A-STRUCT 788 10.1016/j.msea.2020.139487 JUN 24 2020
Abstract:
A series of three Al-Cr-Mn-Co-Zr alloys with total alloy contents of 3.58, 4.41 and 4.90 at.% has been produced by blind-die-compaction of gas-atomized nano-composite powders. The alloy microstructures consist of a facecentered cubic (FCC) Al matrix with characteristic distributions of quasicrystalline I-phase dispersoids giving hardness values of up to 162 HV. The consolidated alloys were forged uniaxially to upsets of 30-90% at temperatures of 300-370 degrees C to investigate the effects of deformation processing on the microstructure and mechanical behavior. At low upsets the characteristic phase distribution was retained, whereas at high upsets a refined more homogeneous microstructure developed with a break-up of the coarser I-phase features. No I-phase decomposition was observed for any of the forging conditions studied. Tensile testing of samples cut perpendicular to the forging axis gave yield strengths of up to 480 MPa, and ultimate tensile strengths of up to 520 MPa, but have low ductilities at low upsets. At higher upsets, the samples exhibited moderately reduced strengths but much higher ductilities, with elongations to failure of up to 15%, and reductions in area of up to 50%. An analysis of the strengthening mechanisms indicates that solid solution strengthening by super-saturation of Cr and Mn in the FCC Al matrix may contribute significantly. The modest softening of the alloys at high upset thus probably corresponds to defect-mediated solute redistribution. The corresponding dramatic increase in ductility is attributed to closure of residual porosity from the consolidation process and to the elimination of weakly-bonded particle-particle interfaces that can act as sites for fracture initiation.
Update: 2-Jul-2020
Title:
Slave-boson analysis of the two-dimensional Hubbard model
Authors:
Riegler, D, Klett, M, Neupert, T, Thomale, R, Wolfle, P
Source:
PHYS REV B 101 (23) 10.1103/PhysRevB.101.235137 JUN 15 2020
Abstract:
We present a comprehensive study of the two-dimensional one-band Hubbard model applying the spin-rotation-invariant slave-boson method. We utilize a spiral magnetic mean field and fluctuations around a paramagnetic mean field to determine the magnetic phase diagram and find the two approaches to be in good agreement. Apart from the commensurate phases characterized by ordering wave vectors Q = (pi, pi), (0, pi ), and (0,0) we find incommensurate phases where the ordering wave vectors Q = (Q, Q) and (Q, pi) vary continuously with filling, interaction strength, or temperature. The mean-field quantities magnetization and effective mass are found to change discontinuously at the phase boundaries separating the (Q, Q) and (Q, pi) phases, indicating a first-order transition. The band structure and Fermi surface is shown in selected cases. The dynamic spin and charge susceptibilities as well as the structure factors are calculated and discussed, including the emergence of collective modes of the zero sound and Mott insulator type. The dynamical conductivity is calculated in dependence of doping, interaction strength, and temperature. Finally, a temperature-interaction strength phase diagram is established.
Title:
Magnetic properties of Cr-substituted epsilon-(Fe1-xCrx)(2)O-3 nanoparticles with epsilon structure
Authors:
Ma, Z, Romaguera, A, Fauth, F, Herrero-Martin, J, Garcia-Munoz, JL, Gich, M
Source:
J MAGN MAGN MATER 506 10.1016/j.jmmm.2020.166764 JUL 15 2020
Abstract:
epsilon-Fe2O3 has gained considerable interest due to its intriguing properties and great application potentials. In this work epsilon-(Fe1-xCrx)(2)O-3 (x = 0, 0.03, 0.075) nanoparticles encapsulated in silica have been successfully prepared by sol-gel chemistry. Synchrotron diffraction confirms that the polar Pna2(1) structure of epsilon-Fe2O3 e is robust against the substitution of Fe by Cr. The incorporation of chromium is accompanied by a softer magnetic behaviour, and also a gradual decrease in the ferromagnetic signal associated to the super-hard FM2 ferrimagnetic phase. While the Cr substitution has little impact on the low temperature incommensurate transition, it is found that Cr substitution shifts the super-hard and soft ferrimagnetic transitions (respectively, T-N2 and T-N1) to notably lower temperatures.
Title:
Magnetic structure and crystalline electric field effects in the triangular antiferromagnet CePtAl4Ge2
Authors:
Shin, S, Pomjakushin, V, Keller, L, Rosa, PFS, Stuhr, U, Niedermayer, C, Sibille, R, Toth, S, Kim, J, Jang, H, Son, SK, Lee, HO, Shang, T, Medarde, M, Bauer, ED, Kenzelmann, M, Park, T
Source:
PHYS REV B 101 (22) 10.1103/PhysRevB.101.224421 JUN 15 2020
Abstract:
Kondo metal CePtAl4Ge2 exhibits long-range antiferromagnetic order below 2.3 K. Neutron powder-diffraction experiments reveal that Ce moments order antiferromagnetically with an incommensurate ordering wave vector k = (1.39, 0, 0.09). Inelastic neutron powder scattering experiments show a magnetic excitation at 14.5 meV, which corresponds to the first excited state due to the crystalline electric field splitting of the ground-state multiplet of Ce3+. The temperature and field dependence of the magnetization of CePtAl4Ge2 is consistent with a doublet ground state with a dominant vertical bar j(z) = 1/2 > character and a first excited doublet vertical bar j(z) = 3/2 > at 14.5 meV.
Title:
The evidence of stacking disorder from dielectric response along the c -axis in the commensurate CDW in bulk 1 T -TaS 2
Authors:
Ma, YC, Wu, D, Wang, ZQ
Source:
SOLID STATE COMMUN 316 10.1016/j.ssc.2020.113946 AUG 2020
Abstract:
The temperature dependent polarization properties along the c-axis of 1T-TaS2 single crystals are investigated. The dielectric constant in MHz region enhances with cooling from 150 K down to 12 K, whereas the spectra disperse only slightly. The dielectric loss spectra at various temperatures indicate a hopping process of localized carriers induced by the out-of-plane stacking disorder. We propose that the enhancement of the polarization with decreasing temperature can be explained by an electrostatic-shielding effect associated with carriers localized in small metallic domains.
Title:
Microstructure and properties evolution of Al-17Si-2Fe alloys with addition of quasicrystal Al-Mn-Ti master alloy
Authors:
Zhang, J, Pang, ZY, Wang, L, Sun, CC, Liu, N, Chen, HM, Cheng, ZX, Li, K
Source:
RARE METALS 39 (10):1210 10.1007/s12598-020-01449-7 OCT 2020
Abstract:
The Fe-rich intermetallic compounds in Al-17Si-2Fe were modified via Al-Mn-Ti quasicrystal master alloy. The effect of master alloy content on the Fe-rich phase morphology was studied by scanning electron microscope (SEM) and thermodynamic calculation. Results show that the microstructure of the Al-Mn-Ti master alloy consists of binary quasicrystal matrix and ternary AlMnTi secondary phase. The evolutive tendency of Fe-rich intermetallic compounds with content of quasi-crystal Al-Mn-Ti master alloy increasing can be described as follows: long needle-shaped beta phase for Al-17Si-2Fe alloy, long plate-shaped ternary delta phase for 3 wt% master alloy addition, Chinese-script and polyhedral alpha phases for 4 wt% master alloy addition and finer plate-shaped quaternary delta phase with alpha phases for 5 wt% master alloy addition. The ultimate tensile strength of the Al-17Si-2Fe alloy with 4 wt% master alloy addition (a mass ratio ofw(Mn)/w(Fe) approximate to 0.7) increases by 23.8% and the friction coefficient decreases from 0.45 to 0.35 compared with those of Mn-free alloy. alpha-Fe phases have less negative effect on the matrix compared with the long needle-shaped beta phase and the plate-shaped delta phase.
Update: 25-Jun-2020
Title:
The modulated low-temperature structure of malayaite, CaSnOSiO4
Authors:
Malcherek, T, Paulenz, B, Fischer, M, Paulmann, C
Source:
ACTA CRYSTALLOGR B 76:316 10.1107/S2052520620003807 JUN 2020
Abstract:
The crystal structure of the mineral malayaite has been studied by single-crystal X-ray diffraction at a temperature of 20 K and by calculation of its phonon dispersion using density functional perturbation theory. The X-ray diffraction data show first-order satellite diffraction maxima at positions q = 0.2606 (8)b*, that are absent at room temperature. The computed phonon dispersion indicates unstable modes associated with dynamic displacements of the Ca atoms. The largest-frequency modulus of these phonon instabilities is located close to a wavevector of q = 0.3b*. These results indicate that the malayaite crystal structure is incommensurately modulated by static displacement of the Ca atoms at low temperatures, caused by the softening of an optic phonon with B-g symmetry.
Title:
LaTe1.82(1): modulated crystal structure and chemical bonding of a chalcogen-deficient rare earth metal polytelluride
Authors:
Poddig, H, Finzel, K, Doert, T
Source:
ACTA CRYSTALLOGR C 76:530 10.1107/S2053229620005094 JUN 2020
Abstract:
Crystals of the rare earth metal polytelluride LaTe1.82(1), namely, lanthanum telluride (1/1.8), have been grown by molten alkali halide flux reactions and vapour-assisted crystallization with iodine. The two-dimensionally incommen-surately modulated crystal structure has been investigated by X-ray diffraction experiments. In contrast to the tetragonal average structure with unit-cell dimensions of a = 4.4996 (5) and c = 9.179 (1) angstrom at 296 (1) K, which was solved and refined in the space group P4/nmm (No. 129), the satellite reflections are not compatible with a tetragonal symmetry but enforce a symmetry reduction. Possible space groups have been derived by group-subgroup relationships and by consideration of previous reports on similar rare earth metal polychalcogenide structures. Two structural models in the orthorhombic superspace group, i.e. Pmmn(alpha, beta, 1/2)000(-alpha, beta,1/2)000 (No. 59.2.51.39) and Pm2(1)n(alpha, beta,1/2)000(-alpha, beta(,)1/2)000 (No. 31.2.51.35), with modulation wave vectors q(1) = alpha a* + beta b* + 1/2c* and q(2) = -alpha a* + beta b* + 1/2c* [alpha = 0.272 (1) and beta = 0.314 (1)], have been established and evaluated against each other. The modulation describes the distribution of defects in the planar [Te] layer, coupled to a displacive modulation due to the formation of different Te anions. The bonding situation in the planar [Te] layer and the different Te anion species have been investigated by density functional theory (DFT) methods and an electron localizability indicator (ELI-D)-based bonding analysis on three different approximants. The temperature-dependent electrical resistance revealed a semiconducting behaviour with an estimated band gap of 0.17 eV.
Title:
Enhancement of resistivity and magnetization of Bi1-xLaxFe1-yMnyO3 ceramics by composition optimization
Authors:
Carvalho, TT, Manjunath, B, de la Cruz, JP, Amaral, VS, Fernandes, JRA, Almeida, A, Moreira, JA, Vilarinho, R, Tavares, PB
This work aims at studying the effect of La and Mn substituents on the structural, electrical and magnetic properties of Bi1-xLaxFe1-yMnyO3 (0 <= x <= 0.30; 0 <= y <= 0.20) at room temperature, in order to find out the optimal compositions that provide both high resistivity and remnant magnetization. The analysis of the XRD patterns and Raman spectra suggest a progressive transition from a rhombohedral for x < 0.15 with y = 0, and x = 0 with y <= 0.20 to an orthorhombic structure for x + y>0.20. Moreover, we observe satellite peaks associated with an incommensurate modulated (IM) orthorhombic structure for x >= 0.15 with y = 0, and x = 0.20 with y = 0.10. We were able to achieve a decrease by several orders of magnitude of the leakage current density and the emergence of a weak ferromagnetic response in the range of compositions 0.10< x <= 0.3 and 0
Title:
Second harmonic generation as a probe of broken mirror symmetry
Authors:
Fichera, BT, Kogar, A, Ye, LD, Gokce, B, Zong, A, Checkelsky, JG, Gedik, N
Source:
PHYS REV B 101 (24) 10.1103/PhysRevB.101.241106 JUN 8 2020
Abstract:
The notion of spontaneous symmetry breaking has been used to describe phase transitions in a variety of physical systems. In crystalline solids, the breaking of certain symmetries, such as mirror symmetry, is difficult to detect unambiguously. Using 1T-TaS2, we demonstrate here that rotational-anisotropy second harmonic generation (RA-SHG) is not only a sensitive technique for the detection of broken mirror symmetry, but also that it can differentiate between mirror symmetry-broken structures of opposite planar chirality. We also show that our analysis is applicable to a wide class of different materials with mirror symmetry-breaking transitions. Lastly, we find evidence for bulk mirror symmetry-breaking in the incommensurate charge density wave phase of 1T-TaS2. Our results pave the way for RA-SHG to probe candidate materials where broken mirror symmetry may play a pivotal role.
Title:
Temperature dependences of optical indicatrix and thermal expansion parameters of TlIn(S1-xSex)(2) solid solutions (x=0, 0.02 and 0.06)
Authors:
Say, A, Martynyuk-Lototska, I, Mys, O, Adamenko, D, Kostyrko, M, Gomonnai, O, Vlokh, R
Temperature dependences of relative elongation, thermal expansion coefficient, birefringence increment and angle of optical indicatrix rotation are studied for TlIn(S1-xSex)(2) crystals (x = 0, 0.02 and 0.06). We have found that all of these crystals reveal at least two diffused phase transitions (PTs), from normal phase to incommensurate phase and from incommensurate phase to ferroelectric phase. The temperature regions where these PTs take place are determined. A low-temperature PT represents a diffused PT of the first order. We have detected with our techniques that the solid solution with x = 0.02 is non-uniformly stressed. These internal stresses can be caused by a polytype structure. An additional anomaly in the temperature dependence of thermal expansion coefficient occurring at the temperature T-1 = 214 K in the compound with x = 0.06 also points to the existence of polytype structure.
Title:
Isotropic Chiral Acoustic Phonons in 3D Quasicrystalline Metamaterials
Authors:
Chen, Y, Kadic, M, Guenneau, S, Wegener, M
Source:
PHYS REV LETT 124 (23) 10.1103/PhysRevLett.124.235502 JUN 9 2020
Abstract:
The elastic properties of three-dimensional (3D) crystalline mechanical metamaterials, unlike those of amorphous structures, are generally strongly anisotropic-even in the long-wavelength limit and for highly symmetric crystals. Aiming at isotropic linear elastic wave propagation, we therefore study 3D periodic approximants of 3D icosahedral quasicrystalline mechanical metamaterials consisting of uniaxial chiral metarods. Considering the increasing order of the approximants, we approach nearly isotropic effective speeds of sound and isotropic acoustical activity. The latter is directly connected to circularly polarized 3D metamaterial chiral acoustic phonons-for all propagation directions in three dimensions.
Title:
Quasicrystalline and L1(2) precipitates in a microalloyed Al-Mn-Cu alloy
Authors:
Zupani, F, Gspan, C, Burja, J, Boncina, T
Source:
MATER TODAY COMMUN 22 10.1016/j.mtcomm.2019.100809 MAR 2020
Abstract:
An Al-Mn-Cu alloy was microalloyed with beryllium, scandium and zirconium, cast into a copper mould, and aged at different temperatures. It was characterised using X-Ray Diffraction, Differential Scanning Calorimetry and Transmission Electron Microscopy. The compression tests of samples aged at 400 degrees C for 1 h were performed at room temperature, 300 degrees C and 350 degrees C, while the accelerated creep tests were performed at 300 degrees C and 350 degrees C. Icosahedral Quasicrystalline and L1(2) precipitates prevailed in the as-aged microstructure, while the fraction of decagonal precipitates was low. The precipitates possessed specific orientation relationships with the matrix. The alloy exhibited enhanced hardness, compressive strength, and creep resistance in comparison to the Al-Mn-Cu alloys.
Update: 18-Jun-2020
Title:
Magnetic, thermal, and electronic-transport properties of EuMg2Bi2 single crystals
Authors:
Pakhira, S, Tanatar, MA, Johnston, DC
Source:
PHYS REV B 101 (21) 10.1103/PhysRevB.101.214407 JUN 3 2020
Abstract:
The trigonal compound EuMg2Bi2 has recently been discussed in terms of its topological band properties. These are intertwined with its magnetic properties. Here detailed studies of the magnetic, thermal, and electronic-transport properties of EuMg2Bi2 single crystals are presented. The Eu+2 spins 7/2 in EuMg2Bi2 exhibit an antiferromagnetic (AFM) transition at a temperature T-N = 6.7 K, as previously reported. By analyzing the anisotropic magnetic susceptibility chi data below T-N in terms of molecular-field theory (MFT), the AFM structure is inferred to be a c-axis helix, where the ordered moments in the hexagonal ab-plane layers are aligned ferromagnetically in the ab plane with a turn angle between the moments in adjacent moment planes along the c axis of approximate to 120 degrees. An alternate but less likely magnetic structure is a planar structure with nearest-neighbor Eu spins aligned at approximate to 120 degrees with respect to each other, where these ordered-moment layers are stacked along the c axis. The magnetic heat capacity exhibits a lambda anomaly at T-N with evidence of dynamic short-range magnetic fluctuations both above and below T-N. The high-T limit of the magnetic entropy is close to the theoretical value for spins 7/2. The in-plane electrical resistivity rho(T) data indicate metallic character with a mild and disorder-sensitive upturn below T-min = 23 K. An anomalous rapid drop in rho(T) on cooling below T-N as found in zero field is replaced by a two-step decrease in magnetic fields. The rho(T) measurements also reveal an additional transition below T-N in applied fields of unknown origin that is not observed in the other measurements and may be associated with an incommensurate to commensurate AFM transition. The dependence of T-N on the c-axis magnetic field H-perpendicular to was derived from the field-dependent chi(T),C rho(T), and rho (T) measurements. This T-N (H-perpendicular to) was found to be consistent with the prediction of MFT for a c-axis helix with S = 7/2 and was used to generate a phase diagram in the H-perpendicular to-T plane.
Title:
Incommensurate spin ordering and excitations in multiferroic SrMnGe2O6
Authors:
Colin, CV, Ding, L, Ressouche, E, Robert, J, Terada, N, Gay, F, Lejay, P, Simonet, V, Darie, C, Bordet, P, Petit, S
Source:
PHYS REV B 101 (23) 10.1103/PhysRevB.101.235109 JUN 2 2020
Abstract:
Anisotropic multiferroic properties of SrMnGe2O6 pyroxene single crystals were systematically investigated by means of magnetization, heat capacity, pyroelectric current measurement, and elastic and inelastic neutron-scattering experiments. Single-crystal neutron diffraction allows us to unambiguously reveal the presence of two incommensurate magnetic orderings: a nonpolar amplitude-modulated collinear sinusoidal magnetic structure emerges at T-N1 = 4.36(2) K followed by a polar elliptical cycloidal spin structure below T-N2 = 4.05(2) K. Pyroelectric current measurements on single crystal confirm the appearance of a spontaneous polarization within the (ac) plane below T-N2 associated with the latter magnetic symmetry through the extended Dzyaloshinskii-Moriya mechanism. The magnetic phase diagram was calculated considering the three isotropic exchange couplings relevant in this system. The magnetic excitations spectra of SrMnGe2O6 measured by inelastic neutron scattering were successfully modeled using a set of exchange interactions consistent with this phase diagram.
Title:
Synthesis and Magnon Thermal Transport Properties of Spin Ladder Sr14Cu24O41 Microstructures
Authors:
Chen, X, Kim, J, Jia, QR, Sullivan, SE, Xu, YM, Jarvis, K, Zhou, JS, Shi, L
Source:
ADV FUNCT MATER 30 (30) 10.1002/adfm.202001637 JUL 2020
Abstract:
The spin ladder compound (Sr,Ca,La)(14)Cu24O41 exhibits an incommensurate layered structure with strong antiferromagnetic coupling. Besides intriguing superconducting behavior, recent experiments on bulk Sr14Cu24O41 single crystals have revealed a remarkable magnon thermal conductivity, which is the largest above 100 K among all known quantum magnets. Although bulk (Sr,Ca,La)(14)Cu24O41 crystals have been synthesized and studied extensively, there have been few reports on the synthesis and magnon thermal transport investigation of their microstructures. Here, the synthesis and thermal transport properties of Sr14Cu24O41 microrods are reported. Electron microscopy studies indicate that these microrods synthesized by a coprecipitation method are single crystals grown preferentially along the ladder axis. Based on a four-probe thermal transport measurement, the thermal conductivity of the microrods reveals appreciable magnon transport in the microstructures. According to a kinetic model analysis, magnon transport in the microrods is suppressed mainly by increased point defect scattering compared to the bulk crystals, whereas surface scattering is negligible for anisotropic 1D magnon transport along the ladder. Moreover, the thermal conductivity is enhanced after annealing as a result of reduced oxygen vacancies. These results help to build the foundation for future heterogeneous integration of magnetic microstructures in microscale devices for the transport of energy and quantum information.
Title:
Infinite-pressure phase diagram of binary mixtures of (non)additive hard disks
Authors:
Fayen, E, Jagannathan, A, Foffi, G, Smallenburg, F
Source:
J CHEM PHYS 152 (20) 10.1063/5.0008230 MAY 29 2020
Abstract:
One versatile route to the creation of two-dimensional crystal structures on the nanometer to micrometer scale is the self-assembly of colloidal particles at an interface. Here, we explore the crystal phases that can be expected from the self-assembly of mixtures of spherical particles of two different sizes, which we map to (additive or non-additive) hard-disk mixtures. We map out the infinite-pressure phase diagram for these mixtures using Floppy Box Monte Carlo simulations to systematically sample candidate crystal structures with up to 12 disks in the unit cell. As a function of the size ratio and the number ratio of the two species of particles, we find a rich variety of periodic crystal structures. Additionally, we identify random tiling regions to predict random tiling quasicrystal stability ranges. Increasing non-additivity both gives rise to additional crystal phases and broadens the stability regime for crystal structures involving a large number of large-small contacts, including random tilings. Our results provide useful guidelines for controlling the self-assembly of colloidal particles at interfaces.
Title:
Simulating a quantum commensurate-incommensurate phase transition using two Raman-coupled one-dimensional condensates
Authors:
Kasper, V, Marino, J, Ji, S, Gritsev, V, Schmiedmayer, J, Demler, E
Source:
PHYS REV B 101 (22) 10.1103/PhysRevB.101.224102 JUN 1 2020
Abstract:
We study a transition between a homogeneous and an inhomogeneous phase in a system of one-dimensional, Raman tunnel-coupled Bose gases. The homogeneous phase shows a flat density and phase profile, whereas the inhomogeneous ground state is characterized by periodic density ripples and a soliton staircase in the phase difference. We show that under experimentally viable conditions the transition can be tuned by the wave-vector difference Q of the Raman beams and can be described by the Pokrovsky-Talapov model for the relative phase between the two condensates. Local imaging available in atom chip experiments allows us to observe the soliton lattice directly, while modulation spectroscopy can be used to explore collective modes, such as the phonon mode arising from breaking of translation symmetry by the soliton lattice. In addition, we investigate regimes where the cold atom experiment deviates from the Pokrovsky-Talapov field theory. We predict unusual mesoscopic effects arising from the finite size of the system, such as quantized injection of solitons upon increasing Q, or the system size. For moderate values of Q above criticality, we find that the density modulations in the two gases interplay with the relative phase profile and introduce novel features in the spatial structure of the mode wave functions. Using an inhomogeneous Bogoliubov theory, we show that spatial quantum fluctuations are intertwined with the emerging soliton staircase. Finally, we comment on the prospects of the ultracold atom setup.
Title:
Impurity-pinned incommensurate charge density wave and local phonon excitations in 2H-NbS2
PHYS REV B 101 (24) 10.1103/PhysRevB.101.241404 JUN 1 2020
Abstract:
Here, we report a scanning tunneling microscopy (STM) and spectroscopy (STS) study in the superconducting state of 2H-NbS2. We directly visualize the existence of an incommensurate charge density wave (CDW) that is pinned by atomic impurities. In strong tunneling conditions, the incommensurate CDW is depinned from impurities by the electric field from the STM tip. To explore the possible mechanism of this incommensurate CDW, we perform STM-based inelastic tunneling spectroscopy (IETS) to detect phonon excitations in 2H-NbS2 and measure the influence of atomic impurities on local phonon excitations. In comparison with the calculated vibrational density of states in 2H-NbS2, we find two branches of phonon excitations which correspond to the vibrations of Nb ions and S ions, and the strength of the local phonon excitations is insensitive to the atomic impurities. Our results demonstrate the coexistence of impurity-pinned incommensurate CDW and superconductivity in 2H-NbS2, and open a way for detecting atomic-scale phonon excitations in transition metal dichalcogenides with STM-based IETS.
Title:
Antimicrobial behavior of leached Al-Cu-Fe-based quasicrystals
Authors:
Zahoor, A, Aziz, T, Zulfiqar, S, Sadiq, A, Ali, R, Shahid, RN, Tariq, NU, Shah, A, Shehzad, K, Ali, F, Bin Awais, H
Source:
APPL PHYS A-MATER 126 (6) 10.1007/s00339-020-03611-5 MAY 20 2020
Abstract:
In this study, for the first time, antimicrobial properties of Al-Cu-Fe, Al-Cu-Fe-B and Al-Cu-Fe-Co quasicrystal powders were investigated in the leached and un-leached condition against Gram-negative (E. aerogenes, K. pneumoniae) and Gram-positive (B. cereus, K. rosea) bacterial environment. Leaching of the powders in 10 M NaOH aqueous solution resulted in the enrichment of Cu and Fe at the surface. Consequently, bacterial activities in the vicinity of the leached quasicrystal powders were inhibited, indicating good antimicrobial characteristics of the leached powders. All the three leached powder samples exhibited antimicrobial activities with a varying degree. From the diameter of inhibition zone, it was deduced that E. aerogenes are the most susceptible against the leached powders. The leached Al-Cu-Fe-B and Al-Cu-Fe-Co quasicrystal powders showed nanostructured features on the outer surface. During leaching, the icosahedral structure was retained in all the samples.
Update: 11-Jun-2020
Title:
Quasiperiodic Crystals: Teaching Aperiodicity of a Crystal Lattice with 3D-Printed Penrose Tiles
Authors:
Rossi, S, Rossi, C, Accorigi, N
Source:
J CHEM EDUC 97 (5):1391 10.1021/acs.jchemed.9b00702 MAY 12 2020
Abstract:
In this paper, a teaching approach for the comprehension of the symmetry properties of an aperiodic crystal structure was described. Building a continuous-fill (quasi)-crystalline pattern with the support of 3D-printed Penrose tiles, students can grasp the lack of translational symmetry typical of an aperiodic crystal.
Title:
Influence of Molecular Orbitals on Magnetic Properties of FeO2Hx
Authors:
Shorikov, AO, Skornyakov, SL, Anisimov, VI, Streltsov, SV, Poteryaev, AI
Source:
MOLECULES 25 (9) 10.3390/molecules25092211 MAY 2020
Abstract:
Recent discoveries of various novel iron oxides and hydrides, which become stable at very high pressure and temperature, are extremely important for geoscience. In this paper, we report the results of an investigation on the electronic structure and magnetic properties of the hydride FeO2Hx, using density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations. An increase in the hydrogen concentration resulted in the destruction of dimeric oxygen pairs and, hence, a specific band structure of FeO2 with strongly hybridized Fe-t(2g)-O-p(z) anti-bonding molecular orbitals, which led to a metallic state with the Fe ions at nearly 3+. Increasing the H concentration resulted in effective mass enhancement growth which indicated an increase in the magnetic moment localization. The calculated static momentum-resolved spin susceptibility demonstrated that an incommensurate antiferromagnetic (AFM) order was expected for FeO2, whereas strong ferromagnetic (FM) fluctuations were observed for FeO2H.
Title:
Mechanically driven structural transformation in Sn reinforced Al -Cu -Fe quasicrystalline matrix nanocomposite
Authors:
Shadangi, Y, Shivam, V, Varalakshmi, S, Basu, J, Chattopadhyay, K, Majumdar, B, Mukhopadhyay, NK
In the present investigation efforts were made to study mechanically driven structural transformation in Sn reinforced AleCueFe icosahedral quasicrystalline (IQC) matrix nanocomposites. The sequence of structural transformation, phase composition, thermal stability and hardness of mechanically milled IQCSn nanocomposite powder were studied through X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential scanning caloriemtery (DSC) and nanoindentation techniques. The XRD result suggests the formation of nano-structured composites. The IQC phase co-existed with monoclinic Al13Fe4 (a = 1.549 nm, b = 0.808 nm, c = 1.248 nm, alpha = beta = 90 degrees, gamma = 107.72 degrees; mC102) and B2-type Al (Cu, Fe) (a = 0.29 nm; cP2) phase in IQC-Sn nanocomposite powder subjected to mechanical milling for 40 h. The structural transformation and fraction of IQC, Al13Fe4 and B2-Al (Cu, Fe) phase depends upon the volume fraction of Sn and duration of milling. The crystalline phases formed during milling transformed to a stable IQC phase along with the other crystalline phases during subsequent annealing treatment. The structural transformations occurring during milling have a remarkable effect on indentation hardness, which is in the range of similar to 4-7 GPa. This suggests that IQC-Sn nanocomposite produced through milling with desirable mechanical properties may be used as potential coating materials for engineering applications. (c) 2020 Elsevier B.V. All rights reserved.
Title:
Development of Al1070-Quasicrystal (Al65Cu23Fe12) composites using friction stir processing and its mechanical characterization
Authors:
Kamalnath, M, Mohan, B, Singh, A, Thirumavalavan, K
Source:
MATER RES EXPRESS 7 (2) 10.1088/2053-1591/ab71c5 FEB 2020
Abstract:
This study aims at analysing the microstructure and mechanical behaviour of quasicrystalline (Al65Cu23Fe12) strengthened Al1070 composites by Friction Stir Processing (FSP). The composites with various volume fractions of quasicrystal are fabricated by a multipass (3 passes) Friction Stir Processing with 1200 rpm speed, 30 mm min(-1) feed rate and tilt angle of 2.5 degrees. The hardness measurement and tensile test performed in the fabricated composites divulged an increasing trend in the hardness values and Ultimate tensile strength up to a volume fraction of 8% of quasicrystals. The microstructural studies disclosed the increase in the hardness values are ascribed to the reduction in grain size due to the dynamic recrystallization and the strengthening of Al matrix by quasicrystals. Though the composites with higher volume fraction exhibited improved Ultimate tensile strength, the fractographic studies depicted there is transformation from ductile to brittle failure in those composites.
Update: 4-Jun-2020
Title:
Application of Combining X-ray Diffraction and Electron Crystallography for Determination of Complex Inorganic Crystal Structure
Authors:
Li, J, Lin, C, Lin, JH, Sun, JL
Source:
ACTA PHYS-CHIM SIN 36 (1) 10.3866/PKU.WHXB201907052 2020
Abstract:
Inorganic, organic, and biological materials have specific natural properties which mostly depend on their atomic structures. The properties of novel materials can be predicted based solely on knowing the structure fully. Thus, structure determination plays a very important role in chemistry, physics, and materials science. X-ray crystallography, including single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD), remains an important technique for studying structures. However, SCXRD can only be applied to high-quality large single crystals without disorders/defects, whereas PXRD provides only one-dimensional information and reflections with the similar d-values will overlap, which makes it difficult to determine the unit-cell parameters, space groups, and accurate intensities. Another important technique for structural determination is electron crystallography (EC). As the electron is the probe, EC alone can be used for those crystals which are too small to be studied by SCXRD or too complex to be studied by PXRD. Electrons interact much more strongly with matter than X-rays; therefore, both electron diffractions (ED) patterns and high-resolution transmission electron microscopy (HRTEM) images can be obtained from nano-sized crystals. Although electron crystallography started later than X-ray crystallography, it has become a very important technique for structural analysis after several decades of development. Especially, three dimensional (3D) ED techniques have been developed, automated electron diffraction tomography (ADT) and rotation electron diffraction (RED), which allow for automated data collection without requiring considerable expertise on the operation of electron microscopes. In addition, the intensities of 3D ED data can be extracted and used for structure determination using specialized software developed for SCXRD. However, the strong interactions between electrons and materials also result in dynamic effects and beam damage. Although the dynamic effects in 3D electron diffraction techniques (ADT and RED) can be significantly reduced, some structures still pose problems for obtaining an initial model due to beam damage. Therefore, EC and X-ray crystallography have significant limitations. For many complicated crystals, a single technique is insufficient to solve the crystal structure and different techniques that supply complementary structural information must be used to obtain a complete structural determination. Herein, the application of X-ray crystallography combined with EC for the analysis of complex inorganic crystal structures will be introduced, covering issues associated with peak overlap, impurities, pseudo-symmetry and twinning, disordered frameworks, location guests, and aperiodic structures.
Title:
An Anisotropic Diluted Magnetic Hybrid Perovskite Series of [CH3NH3][CoxZn1-x(HCOO)(3)]
Authors:
Chen, S, Shang, R, Wang, BW, Wang, ZM, Gao, S
Source:
ACTA PHYS-CHIM SIN 36 (1) 10.3866/PKU.WHXB201907012 2020
Abstract:
Inorganic-organic or hybrid perovskite materials, which are the complementary counterparts of pure inorganic perovskites, can provide many new opportunities in the researches of phase transitions, critical phenomena, and relevant properties, as they combine the characteristics of inorganic and organic components. Therefore, the hybrid perovskites of ammonium metal formate framework are very promising, and their properties have been found to be strongly dependent on the characteristics of the constituent metal ions and/or ammonium ions. Herein, we used solid solution strategies, borrowed from solid state chemistry, to investigate the anisotropic diluted magnetic hybrid perovskite system of [CH3NH3][CoxZn1-x(HCOO)(3)], wherein the B-sites are occupied by the mixed metal ions of Co2+ and Zn2+. The solid solution compounds of this series in the range x = 0-1 (or the molar percent Co% = 0-100%) were successfully prepared using conventional solution chemistry methods. The resulting compounds were demonstrated to be iso-structural by using both single-crystal and powder X-ray diffraction analyses. The solid solution crystals belong to the orthorhombic space group Pnma, with the cell parameters being a = 8.3015(2)-8.3207(3) angstrom, b = 11.6574(4)-11.6811(5) angstrom, c = 8.1315(3)-8.1427(4) angstrom, and V = 787.89(5)-790.98(7) angstrom(3). The perovskite structure consists of a simple cubic anionic metal-formate framework and CH3NH3+ cations which are located in the framework cavities, with N-H center dot center dot center dot O hydrogen bonds formed between the framework and the cation. The members of this series showed negligible changes (< 0.4%) in their respective lattice and structural parameters. Thus, the prepared solid solution compounds constitute good molecule-based examples for the study of magnetic dilution under almost the same structural parameters and molecular geometries. Upon dilution, the magnetization per mole of Co at low temperatures and low fields was suppressed by the magnetic anisotropy of CO2+ and gradual destruction of the large spin canting between coupled Co2+ ions, in contrast to the magnetization enhancement observed in the isotropic diluted system of [CH3NH3][MnxZn1-x(HCOO)(3)] with the same perovskite structure. The percolation limit was estimated as (Co%)(P) = 27(1)% (or xP = 0.27(1)) from the magnetic data, which was slightly lower than that predicted by the percolation theory for a simple cubic lattice (31%); this trend was due to the strong magnetic anisotropy of the present system. In addition, rare incommensurate phase transitions were primarilydetected below similar to 120 K for the pure Co and Zn members, which may also affect the magnetic properties of the materials.
Title:
Superfluid and Supersolid Phases of He-4 on the Second Layer of Graphite
Authors:
Gordillo, MC, Boronat, J
Source:
PHYS REV LETT 124 (20) 10.1103/PhysRevLett.124.205301 MAY 22 2020
Abstract:
We revisited the phase diagram of the second layer of He-4 on top of graphite using quantum Monte Carlo methods. Our aim was to explore the existence of the novel phases suggested recently in experimental works, and determine their properties and stability limits. We found evidence of a superfluid quantum phase with hexatic correlations, induced by the corrugation of the first Helium layer, and a quasi-two-dimensional supersolid corresponding to a 7/12 registered phase. The 4/7 commensurate solid was found to be unstable, while the triangular incommensurate crystals, stable at large densities, were normal.
Title:
Microstructure and Thermal Stability of Icosahedral Quasicrystals in Suction Casting Al-6 at.%Mn(-2 at.%Be) Alloys
Authors:
Chen, ZW, Zhang, Q, Dong, BL, Liu, ZQ
Source:
J MATER ENG PERFORM 29 (1):447 10.1007/s11665-019-04519-9 JAN 2020
Abstract:
Formation and microstructure of icosahedral quasicrystals in suction casting Al-6 at.%Mn(-2 at.%Be) alloys have been investigated by scanning electron microscopy, energy-dispersive spectrometer (EDS), x-ray diffraction (XRD), transmission electron microscopy, and differential scanning calorimetry (DSC). Based on the DSC analysis, isothermal annealing was designed, and XRD was performed to analyze the phase composition after heat treatment. The quasicrystalline microstructure of Al-6 at.%Mn(-2 at.%Be) alloys was obtained by suction casting. Be element addition in Al-Mn alloy modified the appearance of the quasicrystalline phases from hexagons to petals; meanwhile, the primary phase transformed from the Al6Mn compound to quasicrystalline approximant phase. Combined with XRD and scanning results show Be suppressed the formation of Al6Mn and promoted the formation of a quasicrystalline approximant phase. The thermal stability of the icosahedral phases contributes to a better performance of alloys at higher temperature. EDS analysis shows that the quasicrystalline phase is not a stoichiometric compound but a range of compositions.
Title:
Multiple charge density wave states and magnetism in NdPt2Si2 against the background of its nonmagnetic analog LaPt2Si2
Authors:
Falkowski, M, Dolezal, P, Duverger-Nedellec, E, Chamoreau, LM, Forte, J, Andreev, AV, Havela, L
Source:
PHYS REV B 101 (17) 10.1103/PhysRevB.101.174110 MAY 20 2020
Abstract:
Results of structural, magnetic, thermodynamic, and electron transport measurements on single-crystalline NdPt2Si2 are reported. Temperature variations of crystal structure show two different charge density wave states. The structure modulation with the propagation vector q(1) = (0.323, 0, 0) arises below T = 300 K. An additional modulation, with propagation vector q(2) = (0.163, 0.163, 0.500), appears below T = 50 K in a first-order phase transition. This situation was found analogous to isostructural LaPt2Si2 with similar (but not identical) propagation vectors and characteristic temperatures. Nd magnetic moments order antiferromagnetically at T-N = 1.5 K. Temperature dependence of electrical resistivity for current j applied along the a, b, and c axes is strongly affected by the first-order transition, inducing a Fermi surface gapping.
Title:
Dielectric dispersion and ac conductivity behavior in tin-modified lead zirconate antiferroelectric single crystals
Authors:
Jankowska-Sumara, I, Markiewicz, E, Majchrowski, A, Soszynski, A, Podgorna, M
Source:
J APPL PHYS 127 (18) 10.1063/1.5141852 MAY 14 2020
Abstract:
Frequency and temperature dielectric response and electrical conductivity behavior around the antiferroelectric-paraelectric phase transition temperature were studied in PbZr0.93Sn0.07O3 antiferroelectric single crystals. The contribution of conductive processes to dielectric relaxation for the studied frequency range is discussed, considering oxygen vacancies and electronic transport. The relaxation processes below the transition temperature are associated with short-range polaronic hopping due to the interaction of the charge carriers (trapped electrons) with the distorted crystal lattice. Based on the specific coupling between electrons and phonons in the largely distorted oxygen octahedral sublattice of PbZr1-xSnxO3 crystals, an attempt to explain the formation of the incommensurate phase was undertaken.
Title:
Melting behaviour of tri-phasic Bi44In32Sn23 alloy nanoparticle embedded in icosahedral quasicrystalline matrix
Authors:
Tiwari, K, Biswas, K, Palliwal, M, Majumdar, B, Fecht, HJ
Tri-phasic nanoscaled (Bi,In,Sn) alloy nanoparticles (NPs) embedded in the icosahedral quasicrystalline (IQC) matrix were synthesized by the rapid solidification route. The microstructure of the NPs was analyzed using transmission electron microscopy (TEM), which typically reveals the combination of three phases; rhombohedral (Bi), body-centered tetragonal BiIn and hexagonal (g-Sn) in each NP. TEM study also indicates that (Bi) and BiIn bear a reasonably good lattice match with surrounding IQC, whereas (g-Sn) does not develop any specific orientation relationship. In order to understand the melting behavior of tri-phasic NPs, extensive DSC (differential scanning calorimetry), in-situ TEM (transmission electron microscopy) and in-situ XRD (X-ray diffraction) has been carried out. The DSC result reveals substantial depression of melting temperature (similar to 20 degrees C) as compared to bulk. In-situ studies also indicate the solid-state transformation of (gamma-Sn) into (beta-Sn) prior to melting. As the temperature advances, melting initiates at the triple junction of (Bi)/BiIn and matrix. BiIn first melts completely, and the melt front grows into the (b-Sn) phase of the NP. Subsequently, the melt propagates into the interior of (Bi) rich region. The present study provides an insight into the formation of non-equilibrium (gamma-Sn) phase instead of (beta-Sn) and the mechanism of the melting behaviour of three-phase NP embedded in IQC. (c) 2020 Elsevier B.V. All rights reserved.
Title:
Growth and characterisation of single grain Al-Cu-Ru icosahedral quasicrystals from self-fluxes
Authors:
Toyonaga, K, Shibata, R, Yamada, T, de Boissieu, M, Perez, O, Fertey, P, Takakura, H
In order to obtain high-quality single grains of the Al-Cu-Ru icosahedral quasicrystal (iQC), suitable for a structure analysis, the crystal growth conditions with the self-flux method have been studied. The melts of the master alloys with the compositions of Al57.0+xCu39.5-xRu3.5 (x = 0, 2.5, 5, 7.5, 10) and Al62.0Cu34.0+y Ru4.0-y (y = 0, 0.5, 1.5) were held at 1150 degrees C for 2 h, then cooled down to 800, 900, or 1000 degrees C at a rate of -2 K/h, and subsequently retained for various durations, up to 750 h. Single grain iQCs having several millimetre-sizes, which were evaluated their quality by powder X-ray diffraction (XRD), were grown throughout this study. The peak of (664004) reflection in powder XRD of the iQCs grown at 1000 degrees C has approximately 50% narrower width than that grown at 800 degrees C. The inhomogeneity of the compositions intra- as well as inter-grains grown at 800 degrees C was observed. High-quality single grains with homogeneous composition could be achieved with a long-time annealing at 900 degrees C or regardless of the annealing time at 1000 degrees C. By changing the Al/Cu ratio of the master alloys, the composition could also be controlled for the iQCs grown at 1000 degrees C. Single-crystal XRD experiment with synchrotron radiation on Al66.6Cu16.4Ru17.0 iQC, grown at 1000 degrees C, resulted in the collection of 2680 independent Bragg reflections that confirms the high-quality of the sample. The phase retrieval of the diffraction data resulted successfully in obtaining the structure solution, which reveals some characteristic features of this face-centred iQC structure.
Title:
Evolution of porous structure on Al-Cu-Fe quasicrystalline alloy surface and its catalytic activities
In present investigation, the selective removal of Al from the quasi-lattice sites of quasicrystalline alloy surface was examined in order to produce the nano-particles of metal/metal oxides within the microporous network. Al was selectively etched from both the as-cast as well as annealed Al63Cu25Fe12 quasicrystalline alloys through the treatment with 10 mol NaOH solution at different time interval. In the as-cast sample, higher density of porosity was observed compared to that of annealed alloy. However, dealloying specifically for 4 and 8 h yielded nano-size particles on quasicrystalline surface (of both the alloys) in which very fine particles were detected at 8 h. The increase in density and decrease in size of the nano-particles was found with dealloying duration. X-ray diffraction analysis was performed to characterize the samples. Scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray analysis were carried out to investigate the surface microstructure, internal morphology and chemical composition. The chemical dealloying treatments yielded nano-particles of Cu and Fe along with their oxides on the quasicrystalline surface. Furthermore, the catalytic activity of leached quasicrystalline materials was evaluated towards degradation of non-biodegradable and hazardous methylene blue (organic dye). (c) 2020 Elsevier B.V. All rights reserved.
Title:
Fabrication of Al-Cu-Fe particles containing quasicrystalline i-phase by oxidation of omega-phase in air
Authors:
Huang, JR, Yamane, H, Tsai, AP
Source:
J MATER SCI 55 (26):12448 10.1007/s10853-020-04823-z SEP 2020
Abstract:
Alumina-layer-coated spherical particles composed of omega-phase (Al70Cu20Fe10) and icosahedral quasicrystal phase (i-phase, Al64Cu24Fe12) in the Al-Cu-Fe system were fabricated by heating spherical particles of the omega-phase at 700 degrees C for 200-400 h in air. Differential thermal analysis and transmission electron microscopy observations suggested that formation of a liquid phase with a composition close to theta-phase (Al2Cu) was formed near the surface of the particles, and an i-phase domain grew in the interior of the particles through the liquid. When the omega-phase particles were heated at 730 degrees C, which was above the melting point of the omega-phase, for 96-400 h in air, oxide-layer-coated granular particles containing the i-phase as the main phase and lambda-phase (Al13Fe4) and theta-phase as minor phases were obtained. The surface oxide layer acted as a micro-crucible that could hold liquid and prevent cohesion to other particles.
Update: 28-May-2020
Title:
Unexpected magnetic phase in the weakly ordered spin-1/2 chain cuprate Sr2CuO3
PHYS REV B 101 (20) 10.1103/PhysRevB.101.201107 MAY 15 2020
Abstract:
The magnetic phase diagram of a spin-1/2 chain antiferromagnet Sr(2)CuO(3 )is studied by an ultrasound phase-sensitive detection technique. The system is in the extreme proximity of the Luttinger-liquid quantum-critical point and we observe an unusually strong effect of magnetic field, which is very weak compared to the in-chain interaction, on the Neel ordering temperature. Inside the ordered phase, we detect an unexpected, field-induced continuous phase transition. The transition is accompanied by softening of magnetic excitation observed by electron-spin resonance, which in previous work [E. G. Sergeicheva et al., Phys. Rev. B 95, 020411(R) (2017)] was associated with a longitudinal (amplitude) mode of the order parameter. These results suggest a transition from a transverse collinear antiferromagnet to an amplitude-modulated spin-density-wave phase in a very weak magnetic field, which is unexpected for a system of weakly coupled Heisenberg spin-1/2 chains.
Title:
Neutron diffraction study of magnetic ordering in high pressure phases of rare earth metal holmium
Authors:
Perreault, CS, Vohra, YK, dos Santos, AM, Molaison, JJ
Source:
J MAGN MAGN MATER 507 10.1016/j.jmmm.2020.166843 AUG 1 2020
Abstract:
The magnetic ordering in rare earth metals is well established for ambient pressure crystal structures, however, little is known about the magnetic ordering in their corresponding high-pressure crystalline modifications. Holmium (Ho) was studied in a large-volume diamond anvil cell at the Spallation Neutron Source to high-pressure up to 20 GPa and to low-temperature to 10 K. We have conducted two independent high-pressure low-temperature experiments under non-hydrostatic and quasi-hydrostatic pressure conditions respectively. The ambient pressure hexagonal close packed (hcp) phase of holmium shows two magnetic transitions below 10 GPa one to an incommensurate Antiferromagnetic (AFM) phase and another to a conical-Ferromagnetic (c-FM) phase. In contrast, alpha-Samarium-type (a-Sm) phase above 10 GPa and the double hexagonal close packed (dhcp) phase above 19 GPa show only one FM transition marked by the appearance of a magnetic peak at 3 angstrom and the concurrent enhancement of nuclear peaks below 30 K. These new transitions observed by neutron diffraction can be accounted by a commensurate superlattice formation along c-axis in both the a-Sm-type phase and the dhcp phase.
Title:
A(2)SnS(5): A Structural Incommensurate Modulation Exhibiting Strong Second-Harmonic Generation and a High Laser-Induced Damage Threshold (A=Ba, Sr)
ANGEW CHEM INT EDIT 59 (29):11861 10.1002/anie.202004059 JUL 13 2020
Abstract:
Structural modulations have been recently found to cause some unusual physical properties, such as superconductivity or charge density waves; however, thus-induced nonlinear optical properties are rare. We report herein two unprecedented incommensurately modulated nonlinear optical sulfides exhibiting phase matching behavior, A(2)SnS(5) (A=Ba, Sr), with the (3+1)D superspace groups P2(1)2(1)2(00 gamma)00s or P2(1)(alpha 0 gamma)0, featuring different modulations of the [Sn2S7](infinity) belts. Remarkably, Ba2SnS5 exhibits an excellent second harmonic generation (SHG) of 1.1 times that of the benchmark compound AgGaS2 at 1570 nm and a very large laser-induced damage threshold (LIDT) of 8xAgGaS(2). Theoretical studies revealed that the structural modulations increase the distortions of the Sn/S building units by about 44 or 25 % in A(2)SnS(5) (A=Ba, Sr), respectively, and enhance significantly the SHG compared with alpha-Ba2SnSe5 without modulation. Besides, despite the smaller E-g, the A(2)SnS(5) samples exhibit higher LIDTs owing to their smaller thermal expansion anisotropies (Ba2SnS5 (1.51)
Title:
A decagonal quasicrystal with rhombic and hexagonal tiles decorated with icosahedral structural units
Authors:
Wang, WZ, Zhou, XZ, Yang, ZQ, Qi, Y, Ye, HQ
Source:
IUCRJ 7:535 10.1107/S2052252520004297 MAY 2020
Abstract:
The structure of a decagonal quasicrystal in the Zn58Mg40Y2 (at. %) alloy was studied using electron diffraction and atomic resolution Z-contrast imaging techniques. This stable Frank-Kasper Zn-Mg-Y decagonal quasicrystal has an atomic structure which can be modeled with a rhombic/hexagonal tiling decorated with icosahedral units at each vertex. No perfect decagonal clusters were observed in the Zn-Mg-Y decagonal quasicrystal, which differs from the Zn-Mg-Dy decagonal crystal with the same space group P10/mmm. Y atoms occupy the center of 'dented decagon' motifs consisting of three fat rhombic and two flattened hexagonal tiles. About 75% of fat rhombic tiles are arranged in groups of five forming star motifs, while the others connect with each other in a 'zigzag' configuration. This decagonal quasicrystal has a composition of Zn68.3Mg29.1Y2.6 (at. %) with a valence electron concentration (e/a) of about 2.03, which is in accord with the Hume-Rothery criterion for the formation of the Zn-based quasicrystal phase (e/a = 2.0-2.15).
Title:
Ferromagnetic 2/1 quasicrystal approximants
Authors:
Inagaki, K, Suzuki, S, Ishikawa, A, Tsugawa, T, Aya, F, Yamada, T, Tokiwa, K, Takeuchi, T, Tamura, R
Source:
PHYS REV B 101 (18) 10.1103/PhysRevB.101.180405 MAY 13 2020
Abstract:
The existence of various magnetic orders has recently been established in the Tsai-type 1/1 approximant Au-Al-Gd by variation of the electron-per-atom (e/a) ratio [Ishikawa et al. Phys. Rev. B 98, 220403(R) (2018)]. Here, we report ferromagnetic (FM) 2/1 quasicrystal approximants and show that the magnetic order of higher-order approximants can be tailored starting from known magnetic 1/1 approximants. The (Au,Cu)-(Al,In)-R (R = Gd, Tb) 2/1 approximants are synthesized by the simultaneous substitution of isovalent elements Cu and In for Au and Al, respectively, to the FM Au-Al-R (R = Gd, Tb) 1/1 approximants with e = 1.74. Both the (Au,Cu)-(Al,In)-R (R = Gd, Tb) 2/1 approximants exhibit a FM transition at significantly high Curie temperatures of T-C = 30.0 and 15.3 K, respectively. Therefore, this method of isovalent substitution will also enable the realization of a variety of magnetic orders for the 2/1 approximant by variation of the e/a ratio. Such isovalent substitution may also stabilize magnetic quasicrystals for a given e/a ratio, which would lead to the realization of the long-range magnetic order in quasicrystals.
Title:
Partial antiferromagnetic helical order in single-crystal Fe3PO4O3
Authors:
Sarkis, CL, Tarne, MJ, Neilson, JR, Cao, HB, Coldren, E, Gelfand, MP, Ross, KA
Source:
PHYS REV B 101 (18) 10.1103/PhysRevB.101.184417 MAY 18 2020
Abstract:
Magnetic frustration in Fe3PO4O3 produces an unusual magnetic state below T-N = 163 K, where incommensurate antiferromagnetic order is restricted to nanosized needle-like domains, as inferred from neutron powder diffraction. Here we show using single-crystal neutron diffraction that Fe3PO4O3 does not exhibit a preferred ordering wave vector direction in the ab plane despite having a well-defined ordering wave vector length. This results in the observation of continuous rings of scattering rather than satellite Bragg peaks. The lack of a preferred incommensurate ordering wave vector direction can be understood in terms of an antiferromagnetic Heisenberg model with nearest-neighbor (J(1)) and second-neighbor (J(2)) interactions, which produce a quasidegenerate manifold of ordering wave vectors. This state appears to be similar to the partially ordered phase of MnSi, but in Fe3PO4O3 arises in a frustrated antiferromagnet rather than a chiral ferromagnet.
Title:
Magnetic states and electronic properties of Sr4V2O6Fe2As2 studied by DFT calculations
Authors:
Mai, TL, Tran, VH
Source:
COMP MATER SCI 179 10.1016/j.commatsci.2020.109676 JUN 15 2020
Abstract:
In this work, we examine the magnetic stability of two incommensurate antiferromagnetic configurations and electronic band structures properties of Sr4V2O6Fe2As2 using first-principle density-functional-theory method. We show that the lowest total energy takes place in an incommensurate antiferromagnetic phase with the ordering wave vector q = (0.125, 0.125, 0) and ordered magnetic moment of 1.25 mu(B)/V. Based on the total as well as partial density of states (DOS) data, it concludes that both Fe-3d and V-3d orbitals give a rise to DOS around Fermi level and lead to the occurrence of Van Hove singularity associated with electronic instability. A strong inter-band scattering in the incommensurate anti-ferromagnetic phases is proposed, since this behaviour is followed by a larger number of electronic bands crossing Fermi level as compared to those in nonmagnetic one. Electronic bands and Fermi surface data affirm multi-band structure superconductivity, where electron-like sheets of carriers with low Fermi velocities occur at the corner of the Brillouin zone and hole-like cylinders of carriers with high Fermi velocities situate around the Gamma - Z line. Most importantly, our calculations predict the presence of both vertical and horizontal nodal lines, which seem to be distinct from usual feature of Fe-based superconductors with sole horizontal nodal lines.
Title:
Shock-synthesized quasicrystals
Authors:
Nemeth, P
Source:
IUCRJ 7:368 10.1107/S2052252520005254 MAY 2020
Update: 21-May-2020
Title:
High spatial resolution studies of phase transitions within organic aperiodic crystals
Authors:
Marlette, C, Guerin, L, Rabiller, P, Odin, C, Verezhak, M, Bosak, A, Bourges, P, Ecolivet, C, Toudic, B
Source:
PHYS REV B 101 (18) 10.1103/PhysRevB.101.184107 MAY 11 2020
Abstract:
The understanding of the symmetry breakings within crystals that are aperiodic by construction is actually very limited. Quasicrystals and incommensurate composite crystals may potentially allow such studies. We focus on the phase transitions of the aperiodic n-nonadecane/urea which recovers a translational symmetry within a four-dimensional space at room temperature. High-resolution neutron and synchrotron studies are reported as a function of the temperature on this organic crystal which presents an exceptional mosaicity. They reveal the richness of such approach, showing the appearance of very long wavelength supplementary intermodulations. This work generalizes the Landau theory to incommensurate composite crystals.
Title:
Single-Crystal Growth and Physical Properties of EuZn2Ge2
Authors:
Kosaka, M, Michimura, S, Hirabayashi, H, Numakura, R, Iizuka, R, Kuwahara, K, Uwatoko, Y
Source:
J PHYS SOC JPN 89 (5) 10.7566/JPSJ.89.054704 MAY 15 2020
Abstract:
In this study, single crystals of EuZn2Ge2 were grown Itom Ph Ilux. The structure, as determined from single-crystal X-ray diffraction (XRD) data, was ThCr2Si2-type, I4/minm, a = 4.34210(10), c = 10.5766(5)angstrom. The magnetic, thermal, and electronic transport properties of the single crystals were investigated based on their magnetic susceptibility chi(T), isothermal magnetization M(H), specific heat C(T), and electrical resistivity rho(T) measurements, The EuZn2Ge2 compound contains divalent europium and orders antiferromagnetically at T-N1 = 14.9K with a second-order phase transition. In addition, successive transitions were observed at T-N2 = 8.4 K and T-t similar to 5K. At T-N2, the C(T) and chi(T) data indicated a sharp peak, which resembled a lirst-order transition hut exhibited no hysteresis. To verify Whether a structural phase transition occurs at T-N2, we performed low -temperature single -crystal XRD measurements. The XRD pattern did not indicate any structural transition in the crystal lattice, from the tetragonal to a lower symmetry, Instead, changes in the lattice parameter deviations Delta a/a and Delta c/c of the order of 6 x 10(-5), associated with the magnetostriction effect, were observed at approximately T-N2, presumably owing to a magnetic order-order transition from an incommensurate to a commensurate magnetic structure.
Title:
The Subchalcogenides Ir(2)In(8)Q (Q = S, Se, Te): Dirac Semimetal Candidates with Re-entrant Structural Modulation
J AM CHEM SOC 142 (13):6312 10.1021/jacs.0c00809 APR 1 2020
Abstract:
Subchalcogenides are uncommon compounds where the metal atoms are in unusually low formal oxidation states. They bridge the gap between intermetallics and semiconductors and can have unexpected structures and properties because of the exotic nature of their chemical bonding as they contain both metal-metal and metal-main group (e.g., halide, chalcogenide) interactions. Finding new members of this class of materials presents synthetic challenges as attempts to make them often result in phase separation into binary compounds. We overcome this difficulty by utilizing indium as a metal flux to synthesize large (millimeter scale) single crystals of novel subchalcogenide materials. Herein, we report two new compounds Ir(2)In(8)Q (Q = Se, Te) and compare their structural and electrical properties to the previously reported Ir2In8S analogue. Ir2In8Se and Ir2In8Te crystallize in the P4(2)/mnm space group and are isostructural to Ir2In8S, but also have commensurately modulated (with q vectors q = 1/6a* + 1/6b* and q = 1/10a* + 1/10b* for Ir2In8Se and Ir2In8Te, respectively) low-temperature phase transitions, where the chalcogenide anions in the channels experience a distortion in the form of In-Q bond alternation along the ab plane. Both compounds display re-entrant structural behavior, where the supercells appear on cooling but revert to the original subcell below 100 K, suggesting competing structural and electronic interactions dictate the overall structure. Notably, these materials are topological semimetal candidates with symmetry-protected Dirac crossings near the Fermi level and exhibit high electron mobilities (similar to 1500 cm(2) V-1 s(-1) at 1.8 K) and moderate carrier concentrations (similar to 10(20) cm(-3)) from charge transport measurements. This work highlights metal flux as a synthetic route to high quality single crystals of novel intermetallic subchalcogenides with Dirac semimetal behavior.
Title:
Magnetic phase boundary of BaVS3 clarified with high-pressure mu+SR
Authors:
Sugiyama, J, Andreica, D, Forslund, OK, Nocerino, E, Matsubara, N, Sassa, Y, Guguchia, Z, Khasanov, R, Pratt, FL, Nakamura, H, Mansson, M
Source:
PHYS REV B 101 (17) 10.1103/PhysRevB.101.174403 MAY 4 2020
Abstract:
The magnetic nature of the quasi-one-dimensional BaVS3 has been studied as a function of temperature down to 0.25 K and pressure up to 1.97 GPa on a powder sample using the positive muon spin rotation and relaxation (mu(+) SR) technique. At ambient pressure, BaVS3 enters an incommensurate antiferromagnetic ordered state below the Neel temperature (T-N)31 K. T-N is almost constant as the pressure (p) increases from ambient pressure to 1.4 GPa, then T-N decreases rapidly for p > 1.4 GPa, and finally disappears at p similar to 1.8 GPa, above which a metallic phase is stabilized. Hence, T-N is found to be equivalent to the pressure-induced metal-insulator transition temperature (T-MI) at p > 1.4 GPa.
Title:
Horizontal Line Nodes in Sr2RuO4 Proved by Spin Resonance
Authors:
Iida, K, Kofu, M, Suzuki, K, Murai, N, Ohira-Kawamura, S, Kajimoto, R, Inamura, Y, Ishikado, M, Hasegawa, S, Masuda, T, Yoshida, Y, Kakurai, K, Machida, K, Lee, S
Source:
J PHYS SOC JPN 89 (5) 10.7566/JPSJ.89.053702 MAY 15 2020
Abstract:
We investigated the low-energy incommensurate (IC) magnetic fluctuations in Sr2RuO4 by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. We observed a spin resonance with energy of h omega(res) = 0.56meV centered at a characteristic wavevector Q(res) = o0:3; 0:3; 0:5THORN. The resonance energy corresponds well to the superconducting gap 2 Delta = 0.56meV estimated by the tunneling spectroscopy. The spin resonance shows the L modulation with a maximum at around L = 0.5. The L modulated intensity of the spin resonance and our RPA calculations indicate that the superconducting gaps regarding the quasi-one-dimensional alpha and beta sheets at the Fermi surfaces have the horizontal line nodes. These results may set a strong constraint on the pairing symmetry of Sr2RuO4. We also discuss the implications on possible superconducting order parameters.
Title:
Solidification of Metallic Alloys: Does the Structure of the Liquid Matter?
Authors:
Rappaz, M, Jarry, PH, Kurtuldu, G, Zollinger, J
Source:
METALL MATER TRANS A 51 (6):2651 10.1007/s11661-020-05770-9 JUN 2020
Abstract:
In 1952, Frank (Proc R Soc Lond Ser-Math Phys Sci 215:43-46, 1952) already postulated that Icosahedral Short Range Order (ISRO) of atoms in the liquid could possibly explain the large nucleation undercoolings measured in metallic alloys by Turnbull and Fisher (J Chem Phys 17:71-73, 1949). About thirty years later, this conjecture was proven to be key for the understanding of Quasicrystals (QC) formation (Shechtman et al. in Phys Rev Lett 53:20, 1951-3, 1984). More recently, it has been found that a few tens to thousand ppm of solute elements in Al-base and Au-base alloys can influence the nucleation and growth of the primary fcc phase via mechanisms involving ISRO and QC formation. ISRO has also been found to limit the mobility, and thus diffusion, of atoms in the liquid. This can lead to out-of-equilibrium conditions, e.g., the formation of metastable phases or supersaturated solid solution, at reduced velocity compared to alloys where ISRO is not predominantly present. Finally, there are several experimental evidences that ISRO is also responsible for twinned dendrites formation in Al alloys. The present contribution summarizes these recent findings and points out the implications that these might have in the field of solidification and additive manufacturing.
Update: 14_May_2020
Title:
Entropy-Driven Incommensurability: Chemical Pressure-Guided Polymorphism in PdBi and the Origins of Lock-In Phenomena in Modulated Systems
Authors:
Folkers, LC, Warden, HEM, Fredrickson, DC, Lidin, S
Incommensurate order, in which two or more mismatched periodic patterns combine to make a long-range ordered yet aperiodic structure, is emerging as a general phenomenon impacting the crystal structures of compounds ranging from alloys and nominally simple salts to organic molecules and proteins. The origins of incommensurability in these systems are often unclear, but it is commonly associated with relatively weak interactions that become apparent only at low temperatures. In this article, we elucidate an incommensurate modulation in the intermetallic compound PdBi that arises from a different mechanism: the controlled increase of entropy at higher temperatures. Following the synthesis of PdBi, we structurally characterize two low-temperature polymorphs of the TII-type structure with single crystal synchrotron X-ray diffraction. At room temperature, we find a simple commensurate superstructure of the TII-type structure (comm-PdBi), in which the Pd sublattice distorts to form a 2D pattern of short and long Pd-Pd contacts. Upon heating, the structure converts to an incommensurate variant (incomm-PdBi) corresponding to the insertion of thin slabs of the original TII type into the superstructure. Theoretical bonding analysis suggests that comm-PdBi is driven by the formation of isolobal Pd-Pd bonds along shortened contacts in the distorted Pd network, which is qualitatively in accord with the 18-n rule but partially frustrated by the population of competing Bi-Bi bonding states. The emergence of incomm-PdBi upon heating is rationalized with the DFT-Cemical Pressure (CP) method: the insertion of TII-type slabs result in regions of higher vibrational freedom that are entropically favored at higher temperatures. High-temperature incommensurability may be encountered in other materials when bond formation is weakened by competing electronic states, and there is a path for accommodating defects in the CP scheme.
Title:
Synthesis and Electron Microscopy Study of the Quaternary Misfit Layer Chalcogenides {(Bi,Nd)S}(1+delta)CrS2 and {(Pb,Nd)Se}(1+delta)(NbSe2)(2)
Authors:
Varade-Lopez, R, Gomez-Herrero, A, Avila-Brande, D, Otero-Diaz, LC
In the present work, two compounds, in the Bi-Nd-Cr-S and Pb-Nd-Nb-Se systems, not reported to date were synthesized. The chemical composition and the structural determination of these complex compounds, at atomic resolution, was performed through conventional and aberration-corrected electron microscopy including selected area electron diffraction, high resolution (HR) transmission electron microscopy (TEM), HR scanning TEM, and the analytical associated techniques X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy. The average compositions are [(Bi-0.4,Nd-0.6)S](1.)25CrS2 and [(Pb-0.5,Nd-0.5)Se](1.15)(Nb1.0Se2)(2), respectively. By using these electron microscopy techniques, we confirmed that both compounds can be described in term of two interpenetrated sublattices that fit along a but do not fit along b, giving rise to an incommensurate modulation. A closer inspection along the stacking direction of the subcell has provided an ideal structural model for [(Bi-0.4,Nd-0.6)S](1.)25CrS2 based on the intergrowth of one layer of CrS2, three atoms thick, (111) B1 type, and one layer of (Bi, Nd)Se, two atoms thick, (100) B1 type. In [(Pb-0.5,Nd-0.5)Se](1.15)(Nb1.0Se2)(2 )we found that two layers of NbSe2, which adopt the 2H-NbSe2 polytype, alternate with one layer of (Pb, Nd)Se B1 type. In addition, crystals showing extended defects, associated with the weak interaction between the layers, were frequently found.
Title:
Helical and collinear spin density wave order in the S=1/2 one-dimensional frustrated chain compound NaCuMoO4(OH) investigated by neutron scattering
Authors:
Asai, S, Oyama, T, Nawa, K, Nakao, A, Munakata, K, Kuwahara, K, Hagihala, M, Itoh, S, Hiroi, Z, Masuda, T
Source:
PHYS REV B 101 (14) 10.1103/PhysRevB.101.144437 APR 28 2020
Abstract:
We performed neutron diffraction experiments on a single crystal of an S = 1/2 frustrated ferromagnetic chain compound NaCuMoO4(OH) at zero and finite magnetic fields. Three magnetic Bragg peaks with the propagation vector of q = (0, delta, 0) [delta = 0.481(1)] were observed at the zero field, which means that an incommensurate helical magnetic order was realized. Using the ratio of the intensities between these magnetic peaks, a proper-screw structure was suggested, where the spiral plane is the crystallographic ac plane. While the delta is unchanged at lower fields, it decreases linearly with the field where mu H-0 >= 2.5 T. The field dependence is consistent with that of the field-induced spin density wave state that originated from the formation of the bound state of two magnons in the S = 1/2 frustrated ferromagnetic chain.
Title:
Quantum cluster quasicrystals
Authors:
Pupillo, G, Ziherl, P, Cinti, F
Source:
PHYS REV B 101 (13) 10.1103/PhysRevB.101.134522 APR 29 2020
Abstract:
Quasicrystals remain among the most intriguing materials in physics and chemistry. Their structure results in many unusual properties, including anomalously low friction as well as poor electrical and thermal conductivity, but it also supports superconductivity, which shows that quantum effects in quasicrystals can be quite unique. We theoretically study superfluidity in a model quantum cluster quasicrystal. Using path-integral Monte Carlo simulations, we explore a two-dimensional ensemble of bosons with the Lifshitz-Petrich-Gaussian pair potential, finding that moderate quantum fluctuations do not destroy the dodecagonal quasicrystalline order. This quasicrystal is characterized by a small yet finite superfluidity, demonstrating that particle clustering combined with the local cogwheel structure can underpin superfluidity even in the almost classical regime. This type of distributed superfluidity may also be expected in certain open crystalline lattices. Large quantum fluctuations are shown to induce transitions to cluster solids, supersolids, and superfluids, which we characterize fully quantum mechanically.
Title:
EBSD orientation analysis based on experimental Kikuchi reference patterns
Authors:
Winkelmann, A, Cios, G, Tokarski, T, Nolze, G, Hielscher, R, Koziel, T
Source:
ACTA MATER 188:376 10.1016/j.actamat.2020.01.053 APR 15 2020
Abstract:
Orientation determination does not necessarily require complete knowledge of the local atomic arrangement in a crystalline phase. We present a method for microstructural phase discrimination and orientation analysis of phases for which there is only limited crystallographic information available. In this method, experimental Kikuchi diffraction patterns are utilized to generate a self-consistent master reference for use in the technique of Electron Backscatter Diffraction (EBSD). The experimentally derived master data serves as an application-specific reference in EBSD pattern matching approaches. As application examples, we map the locally varying orientations in samples of icosahedral quasicrystals observed in a Ti40Zr40Ni20 alloy, and we analyse AlNiCo decagonal quasicrystals. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.
Title:
Surface wave photonic quasicrystal
Authors:
An, YB, Gao, Z, Ouyang, ZB
Source:
APPL PHYS LETT 116 (15) 10.1063/1.5139267 APR 13 2020
Abstract:
In developing strategies for manipulating surface electromagnetic waves, it has been recently recognized that a complete forbidden bandgap can exist in a periodic surface-wave photonic crystal, which has subsequently produced various surface-wave photonic devices. However, it is not obvious whether such a concept can be extended to a quasi-periodic surface-wave system that lacks translational symmetry. Here, we experimentally demonstrate that a surface-wave photonic quasicrystal that lacks short-range order can also exhibit a forbidden bandgap for surface electromagnetic waves. The lower cutoff of this forbidden bandgap is mainly determined by the maximum separation between the nearest neighboring pillars. Point defects within this bandgap show distinct properties compared to a periodic photonic crystal in the absence of translational symmetry. A line-defect waveguide, which is crafted out of this surface-wave photonic quasicrystal by shortening a random row of metallic rods, is also demonstrated to guide and bend surface waves around sharp corners along an irregular waveguiding path.
Update: 07_May_2020
Title:
Cu6Sn5 intermetallic: Reconciling composition and crystal structure
Authors:
Leineweber, A, Wieser, C, Hugel, W
Source:
SCRIPTA MATER 183:66 10.1016/j.scriptamat.2020.03.020 JUL 1 2020
Abstract:
Cu6Sn5 is the most prominent intermetallic compound developing upon soldering Cu-containing base materials with typical Sn-based lead-free solders. The order-disorder transition occurring at 430-490 K and the possibly associated build-up of stresses is regarded as relevant for reliability of solder joints. In the present work, contradictory reports on the crystal structures and actual composition of the Cu6Sn5 intermetallic are explained by composition-dependent occurrence of the "classical" commensurately ordered eta' phase and of an incommensurately ordered, more Cu-rich eta phase. The previously reported eta(8)-Cu5Sn4 and eta(4+1)-Cu46Sn37 structures with different unit cells and symmetries are actually approximants of the eta phase. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Title:
Nonwetting Behavior of Al-Co Quasicrystalline Approximants Owing to Their Unique Electronic Structures
Authors:
Anand, K, Fournee, V, Prevot, G, Ledieu, J, Gaudry, E
Source:
ACS APPL MATER INTER 12 (13):15793 10.1021/acsami.9b20653 APR 1 2020
Abstract:
Good wetting is generally observed for liquid metals on metallic substrates, while poor wetting usually occurs for metals on insulating oxides. In this work, we report unexpected large contact angles for lead on two metallic approximants to decagonal quasicrystals, namely, Al5Co2 and Al13Co4. Intrinsic surface wettability is predicted from first principles, using a thermodynamic model based on the Young equation, and validated by the good agreement with experimental measurements performed under ultra-high vacuum by scanning electron microscopy. The atomistic details of the atomic and electronic structures at the Pbsubstrate interface, and the comparison with Pb(111)/Al(111), underline the influence of the specific electronic structures of quasicrystalline approximants on wetting. Our work suggests a possible correlation of the contact angles with the density of states at the Fermi energy and paves the way for a better fundamental understanding of wettability on intermetallic substrates, which has potential consequences in several applications such as supported catalysts, protective coatings, or crystal growth.
Title:
Topological quantum phase transitions in one-dimensional pwave superconductors with modulated chemical potentials
Authors:
Wu, JN, Xu, ZH, Lu, ZP, Zhang, YB
Source:
ACTA PHYS SIN-CH ED 69 (7) 10.7498/aps.69.20191868 APR 5 2020
Abstract:
We consider a one-dimensional p-wave superconducting quantum wire with the modulated chemical potential, which is described by (H) over cap = Sigma(i)[(-tc(i)(dagger)c(i+1) + Delta c(i)c(i+1) + h.c.) + V(i)n(i)], V-i = Vcos(2 pi i alpha+delta)/1 - bcos(2 pi i alpha+delta) and can be solved by the Bogoliubov-de Gennes method. When b = 0, alpha is a rational number, the system undergoes a transition from topologically nontrivial phase to topologically trivial phase which is accompanied by the disappearance of the Majorana fermions and the changing of the Z(2) topological invariant of the bulk system. We find the phase transition strongly depends on the strength of potential V and the phase shift delta. For some certain special parameters alpha and delta, the critical strength of the phase transition is infinity. For the incommensurate case, i.e. alpha = (root 5 - 1)/2, the phase diagram is identified by analyzing the low-energy spectrum, the amplitudes of the lowest excitation states, the Z(2) topological invariant and the inverse participation ratio (IPR) which characterizes the localization of the wave functions. Three phases emerge in such case for delta = 0, topologically nontrivial superconductor, topologically trivial superconductor and topologically trivial Anderson insulator. For a topologically nontrivial superconductor, it displays zero-energy Majorana fermions with a Z(2) topological invariant. By calculating the IPR, we find the lowest excitation states of the topologically trivial superconductor and topologically trivial Anderson insulator show different scaling features. For a topologically trivial superconductor, the IPR of the lowest excitation state tends to zero with the increase of the size, while it keeps a finite value for different sizes in the trivial Anderson localization phase.
Title:
Single step fabrication by spray forming of large volume Al-based composites reinforced with quasicrystals
SCRIPTA MATER 181:86 10.1016/j.scriptamat.2020.02.018 MAY 2020
Abstract:
Recently it was shown that the Al-Cu-Fe-Cr system can form a microstructure consisting mainly of quasicrystalline and alpha-Al phases by arc-melting and subsequent annealing of an appropriate composition. In the present work we demonstrate that it is possible to fabricate a large volume of this composite directly from the liquid phase by spray forming an Al85Cu6Fe3Cr6 (%at) alloy. The overspray powder and an arc-melted sample were also analyzed. Tribological properties of the Al-based composite reinforced with quasicrystals was evaluated by pin-on-disk tests, which demonstrated to be superior than an Al-Si alloy, A380, used in this work as a comparison material. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Update: 30_Apr_2020
Title:
Evolution of Structure in the Incommensurate Modulated LaNb1-xWxO4+x/2 (x=0.04-0.16) Oxide Ion Conductors
CHEM MATER 32 (6):2292 10.1021/acs.chemmater.9b04255 MAR 24 2020
Abstract:
Hyper-stoichiometric CeNbO4+d phases demonstrate remarkable oxygen diffusivity and provide an interesting structural template for oxygen ion conductors. Previously, we have reported the room temperature structure of the incommensurate modulated LaNb0.88W0.12O4.06, a structural analogue of CeNbO4+d. We have confirmed that it is a pure oxygen ion conductor, with anions diffusing via an interstitialcy mechanism. However, the high temperature structural information for the LaNb1-xWxO4+d (x = 0.04-0.16) family, which is key to understanding the structure-property relationship in oxygen ionic conductors with complex structures at operating conditions, is unreported. In this contribution, we address this question by investigating the high temperature structural evolution of the LaNb1-xWxO4+2/x phases using a combination of thermal analysis, scattering techniques, and O-17 and Nb-93 nuclear magnetic resonance spectroscopy. We reveal a series of phase transitions between a modulated monoclinic phase, a high temperature modulated tetragonal phase, and a high temperature unmodulated tetragonal phase. These findings are correlated with the ion transport and offer insights into the design of new materials for solid state electrochemical devices.
Title:
Complicated magnetic structure and its strong correlation with the anomalous Hall effect in Mn3Sn
PHYS REV B 101 (14) 10.1103/PhysRevB.101.144422 APR 16 2020
Abstract:
The large anomalous Hall effect (AHE) has recently been found in noncollinear antiferromagnet Mn3Sn. However, the complex magnetic structure and its correlation with the AHE remain unclear. Here we investigate the magnetic structure of Mn3Sn completely by means of both the temperature and magnetic field dependence of neutron power diffraction. This study establishes the possible incommensurate magnetic structure models and extracts the temperature evolution of the magnetic moment of the Mn atom over the whole temperature range for Mn3Sn. The large AHE of polycrystalline bulk Mn3Sn is measured in detail. By comparing the temperature dependence of the spontaneous component of the AHE and the magnetic moment, direct experimental evidence reveals that the triangular antiferromagnetic ordered moment of the Mn atom controls the change in the AHE of Mn3Sn. This study provides the possibility to control the AHE of functional materials via changing the magnetic structure.
Update: 23_Apr_2020
Title:
Magnetic Properties of the RE2Pt6X15 (RE = Y, La-Nd, Sm, Gd-Lu; X = Al, Ga) Series
The members of the RE2Pt6Al15 (RE = Ce-Nd, Sm, Gd-Yb) and RE2Pt6Ga15 (RE = Y, La-Nd, Sm, Gd-Lu) series have been synthesized from the elements via arc-melting followed by annealing in an induction furnace. Isotypism of Ho2Pt6Al15 with orthorhombic (3+1)D commensurately modulated Sc2Pt6Al15 [Cmcm(alpha,0,0)0s0, alpha = 2/3] was observed from single-crystal diffraction data. The diffraction pattern shows the same satellite reflections pattern as the prototype, therefore the same modulated superstructure was refined. Full ordering of the Pt and Al atoms within the [Pt6Al15](delta-) polyanion was observed. The lattice parameters of the averaged hexagonal structure were refined from powder X-ray diffraction experiments and agree well with previous reports on these compounds. The X-ray pure polycrystalline samples of both series were studied with respect to their magnetic properties. The susceptibility data of all investigated compounds show experimental magnetic moments close to the free ion values of the RE3+ cations and antiferro-/ferrimagnetic ordering at low temperatures with Curie temperatures up to T-C = 16.8(1) K for Gd2Pt6Ga15. Both, Sm2Pt6Al15 and Sm2Pt6Ga15 show the typical van Vleck type behavior along with antiferromagnetic ordering at a Neel temperature of T-N = 4.5(1) K (Al) and ferrimagnetic ordering at T-C = 6.5(1) K (Ga). Yb2Pt6Al15 exhibits ytterbium in the trivalent oxidation state, while for Yb2Pt6Ga15 a divalent state of the Yb atoms was observed.
Title:
First-principles calculation of spin and orbital contributions to magnetically ordered moments in Sr2IrO4
PHYS REV B 101 (15) 10.1103/PhysRevB.101.155110 APR 9 2020
Abstract:
We show how an accurate first-principles treatment of the canted-anti-ferromagnetic ground state of Sr2IrO4, a prototypical 5d correlated spin-orbit coupled material, can be obtained without invoking any free parameters, such as the Hubbard U or tuning the spin-orbit coupling strength. Our theoretically predicted iridium magnetic moment of 0.250 mu(B), canted by 12.6 degrees off the a axis, is in accord with experimental results. By resolving the magnetic moments into their spin and orbital components, we show that our theoretically obtained variation of the magnetic scattering amplitude Mm as a function of the polarization angle is consistent with recent nonresonant magnetic x-ray scattering measurements. The computed value of the band gap (55 meV) is also in line with the corresponding experimental values. A comparison of the band structure to that of the cuprates suggests the presence of incommensurate charge-density wave phases in Sr2IrO4.
Title:
Cu-63,Cu-65 NMR study of the magnetically ordered state of the multiferroic CuFeO2
J MAGN MAGN MATER 504 10.1016/j.jmmm.2020.166668 JUN 15 2020
Abstract:
Field-swept Cu-63,Cu-65 NMR spectra under magnetic fields up to 8.3 T at a constant NMR frequency and temperatures T < 12 K on a single crystalline sample of multiferroic CuFeO2 were measured and analyzed. When the magnetic field is applied along the c axis, a nearly zero internal magnetic field at the Cu site in magnetic ordered state was observed. This is explained by the perfect cancellation of the internal fields produced by the 6 nearest neighbor Fe3+ (S = 5/2) ions, revealing the magnetic structure to be a collinear four-sublattice structure. On the other hand, when the magnetic field is applied along the ab plane, we observed a finite internal field at the Cu sites, which is due to the canting of the Fe moments. Strong change in the NMR signal intensity is observed around 7-8 T, corresponding to the magnetic phase transition from the collinear magnetic to ferroelectric incommensurate states. The ratio of the two magnetic phases significantly depends on the history of the change in the external magnetic field and the temperature of the sample. The details of history dependence of the ratio were discussed.
Title:
Fabrication of Al-Cu-Fe quasicrystal reinforced 6082 aluminium matrix nanocomposites through mechanical milling and spark plasma sintering
Authors:
Shadangi, Y, Sharma, S, Shivam, V, Basu, J, Chattopadhyay, K, Majumdar, B, Mukhopadhyay, NK
In the present investigation, attempts were made to study the effect of Al-Cu-Fe (40 vol%) quasicrystalline (QC) reinforcement on the structure, morphology and phase composition of 6082 Al matrix nanocomposites (AMCs) processed through mechanical milling (MM) and spark plasma sintering (SPS). The characterization of these MM and SPSed AMCs was done through X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM). The MM induces microstructural refinement of matrix and partial structural transformation of QC phase to Al13Fe4 approximant phase (a = 1.549 nm, b = 0.808 nm, c = 1.248 nm, alpha = beta = 90 degrees, gamma = 107.72 degrees; mC102; C2/m). The presence of (311111) diffraction peak of the QC phase in AMCs confirms the existence of face-centred QC phase even after 50 h of MM. The consolidation of Al-QC at 450 degrees C (723 K) and 550 degrees C (823 K) results in the fabrication of AMCs having a density of 2.921 and 3.319 g cm(-3) respectively. The compressive yield strength and ultimate strength of these AMCs is similar to 519 MPa and 639 MPa respectively. The enhancement in the mechanical properties may be attributed to strong interfacial bonding of the Al matrix and QC reinforcement due to interfacial reactions. (c) 2020 Elsevier B.V. All rights reserved.
Update: 16_Apr_2020
Title:
Bulk metallic glass formation in the (Ti,Zr)-(Ni,Cu)-S system
Authors:
Gross, O, Ruschel, L, Kuball, A, Bochtler, B, Adam, B, Busch, R
Source:
J PHYS-CONDENS MAT 32 (26) 10.1088/1361-648X/ab7c15 JUN 17 2020
Abstract:
New bulk glass-forming alloy compositions, exceeding a critical casting thickness of 1 mm, are developed in the (quasi-ternary) (Ti,Zr)-(Ni,Cu)-S system. The ternary eutectic composition Ti65.5Ni22.5Cu12 is stepwise modified through additions of S (0-8 at%) and Zr (0-22.5 at%) at the expense of Ni and Ti, respectively. By increasing the plate thickness of the casted samples from 500 mu m to 1.25 mm, the primary precipitating phases are identified which is for the best glass-formers (e.g. Ti58Zr7.5Ni18.5Cu12S4) an icosahedral phase. In calorimetric experiments, several exothermic crystallization events are observed upon heating glassy samples. The first exothermic event, obscuring the glass transition, is attributed to the formation of the icosahedral phase. As the icosahedral phase forms upon heating and cooling for the best glass-formers, the origin of the increased glass-forming ability might be attributed to a pronounced icosahedral short-range order in the liquid state, impeding the formation of the stable crystalline phases.
Update: 9-Apr-2020
Title:
Evidence of charge density wave transverse pinning by x-ray microdiffraction
Authors:
Bellec, E, Gonzalez-Vallejo, I, Jacques, VLR, Sinchenko, AA, Orlov, AP, Monceau, P, Leake, SJ, Le Bolloc'h, D
Source:
PHYS REV B 101 (12) 10.1103/PhysRevB.101.125122 MAR 24 2020
Abstract:
Incommensurate charge density waves (CDW) have the extraordinary ability to display non-Ohmic behavior when submitted to an external field. The mechanism leading to this nontrivial dynamics is still not well understood, although recent experimental studies tend to prove that it is due to solitonic transport. Solitons could come from the relaxation of the strained CDW within an elastic-to-plastic transition. However, the nucleation process and the transport of these charged topological objects have never been observed at the local scale until now. In this paper, we use in situ scanning x-ray microdiffraction with micrometer resolution of a NbSe3 sample designed to have sliding and nonsliding areas. Direct imaging of the charge density wave deformation is obtained using an analytical approach based on the phase gradient to disentangle the transverse from the longitudinal components over a large surface (90 mu m). We show that the CDW dissociates itself from the host lattice in the sliding regime and displays a large transverse deformation, ten times larger than the longitudinal one and strongly dependent on the amplitude and the direction of the applied currents. This deformation continuously extends across the macroscopic sample dimensions, over a distance 10 000 times greater than the CDW wavelength despite the presence of strong defects while remaining strongly pinned by the lateral surfaces. This two-dimensional quantitative study highlights the prominent role of the shear effect, which should be significant in the nucleation of solitons.
Title:
Size, disorder, and charge doping effects in the antiferromagnetic series Eu(1-)(x)A(x)Ga(4) (A = Ca, Sr, or La)
Authors:
Stavinoha, M, Huang, CL, Devlin, KP, Fettinger, JC, Kauzlarich, SM, Morosan, E
Source:
J SOLID STATE CHEM 285 10.1016/j.jssc.2020.121232 MAY 2020
Abstract:
EuGa4 hosts a magnetic Eu2+ sublattice surrounded by a network of covalently-bound Ga atoms with the BaAl4 structure type (space group/4/Trumn). In this study, we present the synthesis and characterization of three new single crystal substitutional series EuA(x)Ga(4) with A = Ca, Sr, or La. X-ray diffraction and resistivity measurements show that Ca substitution induced a structural phase transition from the tetragonal crystal structure at high temperatures to the monoclinic crystal structure (CaGa4 type, space group C2/m) at low temperatures and suppressed the antiferromagnetic ordering temperature to 8.8 K for x = 0.45. Comparatively, La or Sr substitution maintained the tetragonal crystal structure and suppressed the antiferromagnetic ordering temperatures to 6.7 K and 1.6 K for (A, x) = (La, 0.37) and (Sr, 0.91), respectively. In addition to suppressing the magnetic order, magnetization and specific heat measurements indicate the onset of anisotropic metamagnetic transitions in (La, 0.18), (La, 0.37), and (Sr, 0.63), along with an incommensurate-to-commensurate magnetic transition in (Sr, 0.38). By comparing these effects of doping EuGa4, we show how size, disorder, and charge determine the structure-physical property relations in EuGa4 .
Title:
Quasicrystalline phase and crystalline approximant in Ni-Mn-In Heusler alloy system
Intermetallic compounds and micro-structures in Ni-Mn-In alloy system were investigated with transmission electron microscopy, selected-area and nano-beam electron diffraction, and energy-dispersive x-ray spectroscopy. Together with two types of Heusler phases, a decagonal quasicrystalline phase and a structurally related crystallite were found in the Ni-Mn-In system. As is well-known, most of the decagonal quasicrystals are aluminum-based intermetallic phases. It is interesting to investigate the aluminum-free decagonal quasicrystalline phase as a new member of the quasicrystal family. Structural characterization was carried out on the decagonal quasicrystalline phase and the crystalline approximant in the Ni-Mn-In alloy system.
Title:
Magnetic field induced antiferromagnetic cone structure in multiferroic BiFeO3
Authors:
Matsuda, M, Dissanayake, SE, Hong, T, Ozaki, Y, Ito, T, Tokunaga, M, Liu, XZ, Bartkowiak, M, Prokhnenko, O
Source:
PHYS REV MATER 4 (3) 10.1103/PhysRevMaterials.4.034412 MAR 31 2020
Abstract:
Neutron diffraction measurements were performed under high magnetic fields up to 17 T in a multiferroic BiFeO3 single crystal, in which an intermediate magnetic (IM) phase has been found between the cycloid and canted antiferromagnetic phases [S. Kawachi et al., Phys. Rev. Mater. 1, 024408 (2017)]. We clearly found that the incommensurate magnetic peaks, which split perpendicular to the magnetic field in the cycloid phase, rotate by 90 deg to align parallel to the field in the IM phase. The magnetic structure in the IM phase can be best described by an antiferromagnetic cone (AF cone) structure. The transition from the cycloid to AF cone is of first order and the direction of the magnetic wave vector and the easy plane of the cycloidal component are rotated by 90 deg without changing the cycloidal modulation period, whereas the transition from the AF cone to canted antiferromagnetic phase is gradual and the cone angle becomes smaller gradually without changing the modulation period. Interestingly, the cycloidal component as well as the cone angle in the IM phase shows a large hysteresis between the field increasing and decreasing processes. This result, combined with the magnetostriction with a large hysteresis previously reported in the IM phase, suggests a strong magnetoelastic coupling.
Title:
Non-divergent Gruneisen parameter in quantum critical quasicrystal Yb15Al34Au51: Reflection of robustness of quantum criticality
Authors:
Watanabe, S, Miyake, K
Source:
SOLID STATE COMMUN 306 10.1016/j.ssc.2019.113774 FEB 2020
Abstract:
The mechanism of not diverging Gruneisen parameter in the quantum critical heavy-fermion quasicrystal (QC) Yb15Al34Au51 is analyzed. We construct the formalism for calculating the specific heat C-V(T), the thermal-expansion coefficient alpha(T), and the Gruneisen parameter Gamma(T) near the quantum critical point of the Yb valence transition. By applying the framework to the QC, we calculate C-V(T), alpha(T), and Gamma(T), which explains the measurements. Not diverging Gamma(T) is attributed to the robustness of the quantum criticality in the QC under pressure. The difference in Gamma(T) at the lowest temperature between the QC and approximant crystal is shown to reflect the difference in the volume derivative of characteristic energy scales of the critical Yb-valence fluctuation and the Kondo temperature. Possible implications of our theory to future experiments are also discussed.
Title:
STRUCTURAL DISORDER IN QUASICRYSTALS
Authors:
Strzalka, R, Buganski, I, Smietanska, J, Wolny, J
Source:
ARCH METALL MATER 65 (1):291 10.24425/amm.2020.131729 2020
Abstract:
One of the challenges of modem crystallography of complex systems (complex metallic alloys, proteins, aperiodic crystals and quasicrystals) is to properly describe the disorder in these systems and discuss correctly the refinement results in terms of the structural disorder. In this paper we briefly discuss a new approach to phasons and phonons in quasicrystals and focus on the new theory of phonons in these materials. A newly derived correction factor for phonons in the form of the Bessel function is the approximated way of describing optic modes in the phonon spectra of quasicrystals. It is applied to a real decagonal quasicrystal in the Al-Cu-Rh system with 56/38 atoms per thick/thin structural unit, based on 2092 unique reflections selected from the collected diffraction data, significantly improving the refinement results. The final R-factor value is 7.24%, which is over 0.5% better result comparing to originally reported. We believe our work will open a broader discussion on the disorder in quasicrystals (and other aperiodic systems) and motivate to develop new approaches to treat the diffraction data influenced by different types of disorder in the new way.
Title:
Crystal structure and thermoelectric properties of partially-substituted melt-grown higher manganese silicides
Crystal structure and thermoelectric properties of several partially-substituted higher manganese silicides (HMSs) samples have been reviewed. HMSs possess a unique incommensurate crystal structure consisting of two subsystems of [Mn] and [Si] and the structure formula is thus represented as MnSi gamma. The c-axis length ratio, gamma, changes with temperature to yield the MnSi (monosilicide) striations, which deteriorate mechanical strength and electrical conductivity. A small amount of V-substitution effectively dissipates such striations and enhances hole carrier concentration. Thus prepared V-substituted samples exhibit the highest power factor and remarkably lower thermal conductivity. The nanostructure of such samples consists of regular and highly disordered nano-domains of Si atoms, which would further reduce the thermal conductivity to enhance thermoelectric figure-of-merit. (c) 2020 The Japan Society of Applied Physics
Update: 02-Apr-2020 (ma se nacist z hlavicek e-mailu)
Title:
Topological models in rotationally symmetric quasicrystals
Authors:
Duncan, CW, Manna, S, Nielsen, AEB
Source:
PHYS REV B 101 (11) 10.1103/PhysRevB.101.115413 MAR 13 2020
Abstract:
We investigate the physics of quasicrystalline models in the presence of a uniform magnetic field, focusing on the presence and construction of topological states. This is done by using the Hofstadter model but with the sites and couplings denoted by the vertex model of the quasicrystal, giving the Hofstadter vertex model. We specifically consider two-dimensional quasicrystals made from tilings of two tiles with incommensurate areas, focusing on the fivefold Penrose and the eightfold Ammann-Beenker tilings. This introduces two competing scales: the uniform magnetic field and the incommensurate scale of the cells of the tiling. Due to these competing scales, the periodicity of the Hofstadter butterfly is destroyed. We observe the presence of topological edge states on the boundary of the system via the Bott index that exhibit two-way transport along the edge. For the eightfold tiling, we also observe internal edgelike states with nonzero Bott index, which exhibit two-way transport along this internal edge. The presence of these internal edge states is a characteristic of quasicrystalline models which leads to open questions on their properties and future applications. We then move on to considering interacting systems. This is challenging, in part because exact diagonalization on a few tens of sites is not expected to be enough to accurately capture the physics of the quasicrystalline system and in part because it is not clear how to construct topological flatbands having a large number of states. We show that these problems can be circumvented by building the models analytically, and in this way we construct models with Laughlin-type fractional quantum Hall ground states.
Title:
Monovalent (Li+(1)) doping effect in multiferroic GdMnO3
Authors:
Sarkar, R, Sarkar, B, Pal, S
Source:
B MATER SCI 43 (1) 10.1007/s12034-020-2045-4 FEB 10 2020
Abstract:
This paper reports the temperature- and field-dependent magnetic properties of monovalent-doped polycrystalline sample Gd0.85Li0.15MnO3 (GLMO) prepared by conventional solid-state reaction route. Final sintering at 1673 K for 18 h yields in the formation of well-grown, impurity phase free, single-phased, orthorhombic structured (with Pbnm space group) crystal. The optical properties have been investigated by UV-absorption spectra. The room temperature UV-absorption spectrum using Tauc's formula gives an optical band gap of similar to 3.12 eV. The paramagnetic (PM) state to incommensurate antiferromagnetic (ICAFM) state transition temperature increases due to Li doping. Magnetic hysteresis curve at 5 K signifies the Gd spin ordering.
Title:
Magnetic Phase Transitions to an Incommensurate Structure in LiMn2O4 Compound
Authors:
Menshenin, VV
Source:
J EXP THEOR PHYS+ 130 (1):108 10.1134/S1063776119120069 JAN 2020
Abstract:
Possible second-order magnetic phase transitions to the incommensurate magnetic structure in the orthorhombic phase of LiMn2O4 compound have been investigated. It is shown that "weak Lifshitz condition" holds for this compound (i.e., Lifshits invariants are immaterial in all transitions being considered), and only the incommensurate phase, in which the wavevector near the transition point varies continuously with temperature and pressure, can be formed. A transition is considered both in the exchange approximation occurring in three irreducible representations forming an exchange multiplet as well as in one and two irreducible representations. Expressions have been derived for the mean density of the magnetic moment emerging because of such transitions. It is found that structures of the types of a longitudinal spin wave, a transverse spin wave with polarization along the crystallographic axes perpendicular to the wavevector of the structure, as well as certain superpositions of these waves can be formed in the system.
Topological magnon is a vibrant research field gaining more and more attention in the past few years. Among many theoretical proposals and limited experimental studies, ferromagnetic kagome lattice emerges as one of the most elucidating systems. Here we report neutron scattering studies of YMn6Sn6, a metallic system consisting of ferromagnetic kagome planes. This system undergoes a commensurate-to-incommensurate antiferromagnetic phase transition upon cooling with the incommensurability along the out-of-plane direction. We observe magnon band gap opening at the symmetry-protected K points and ascribe this feature to the antisymmetric Dzyaloshinskii-Moriya (DM) interactions. Our observation supports the existence of topological Dirac magnons in both the commensurate collinear and incommensurate coplanar magnetic orders, which is further corroborated by symmetry analysis. This finding places YMn6Sn6 as a promising candidate for room-temperature magnon spintronics applications.
Title:
Proof of the elusive high-temperature incommensurate phase in CuO by spherical neutron polarimetry
Authors:
Qureshi, N, Ressouche, E, Mukhin, A, Gospodinov, M, Skumryev, V
Source:
SCI ADV 6 (7) 10.1126/sciadv.aay7661 FEB 2020
Abstract:
CuO is the only known binary multiferroic compound, and due to its high transition temperature into the multiferroic state, it has been extensively studied. In comparison to other prototype multiferroics, the nature and even the existence of the high-temperature incommensurate paraelectric phase (AF3) were strongly debated-both experimentally and theoretically-since it is stable for only a few tenths of a kelvin just below the Neel temperature. Until now, there is no proof by neutron diffraction techniques owing to its very small ordered Cu magnetic moment. Here, we demonstrate the potential of spherical neutron polarimetry, first, in detecting magnetic structure changes, which are not or weakly manifest in the peak intensity and, second, in deducing the spin arrangement of the so far hypothetic AF3 phase. Our findings suggest two coexisting spin density waves emerging from an accidental degeneracy of the respective states implying a delicate energy balance in the spin Hamiltonian.
Title:
Neutron diffraction and symmetry analysis of the martensitic transformation in Co-doped Ni2MnGa
Authors:
Orlandi, F, Cakir, A, Manuel, P, Khalyavin, DD, Acet, M, Righi, L
Source:
PHYS REV B 101 (9) 10.1103/PhysRevB.101.094105 MAR 18 2020
Abstract:
Martensitic transformations are strain driven displacive transitions governing the mechanical and physical properties in intermetallic materials. This is the case in Ni2MnGa, where the martensite transition is at the heart of the striking magnetic shape memory and magnetocaloric properties. Interestingly, the martensitic transformation is preceded by a premartensite phase, and the role of this precursor and its influence on the martensitic transition and properties is still a matter of debate. In this work we report on the influence of Co doping (Ni50-xCoxMn25Ga25 with x = 3 and 5) on the martensitic transformation path in stoichiometric Ni2MnGa by neutron diffraction. The use of the superspace formalism to describe the crystal structure of the modulated martensitic phases, joined with a group theoretical analysis, allows unfolding the different distortions featuring the structural transitions. Finally, a general Landau thermodynamic potential of the martensitic transformation, based on the symmetry analysis, is outlined. The combined use of phenomenological and crystallographic studies highlights the close relationship between the lattice distortions at the core of the Ni2MnGa physical properties and, more in general, on the properties of the martensitic transformations in the Ni-Mn based Heusler systems.
Title:
Rotational outward solidification casting: An innovative single step process to produce a functionally graded aluminum reinforced with quasicrystal approximant phases
Authors:
Ferreira, T, de Oliveira, IL, Zepon, G, Bolfarini, C
Source:
MATER DESIGN 189 10.1016/j.matdes.2020.108544 APR 2020
Abstract:
A secondary AlSi7.5Cu3-(Fe,Mn) alloy with small additions of Mn and Fe was processed by an innovative rotational outward solidification casting process to produce a functionally graded aluminum (FGA). A cylinder was produced in a single step presenting two well-defined metallurgically bonded layers (A and B) with distinct chemical composition, structure and mechanical properties. Both layers were deeply characterized through different techniques. Layer A microstructure has more than 50%vol. of the primary alpha-Al-12(Fe,Mn,Cr)(3)Si quasicrystal approximant phase embedded in an Al-FCC matrix. On the other hand, layer B presents the typical hypoeutectic microstructure of the conventional AlSi7.5Cu3 alloy. Microhardness of the primary alpha-Al-12(Fe,Mn,Cr)(3)Si phase and macro-hardness profile of the FGA were measured. It was found that the hardness of layer A is approximately 1.6 times higher than layer B. This new FGA may be an alternative for producing parts that require gradient properties such as, for instance, automobile engine blocks. (C) 2020 The Authors. Published by Elsevier Ltd.
Title:
Cone-spiral magnetic ordering dominated lattice distortion and giant negative thermal expansion in Fe-doped MnNiGe compounds
MATER HORIZONS 7 (3):804 10.1039/c9mh01602c MAR 1 2020
Abstract:
Negative thermal expansion (NTE) has emerged as one of the intense research topics to meet the demands of the precision industry for compensating positive thermal expansion (PTE) properties. The adjustment of the NTE behavior is the key for tailoring thermal expansion. Chemical modification and the particle size effect have been regarded as effective means to tune the NTE behavior, and the crystallographic contribution is usually the upper limit of NTE. Here, we reported a new way to tune the NTE behavior involving lattice distortion that is dominated by the magnetic structure in hexagonal MnM'Gebased (M':Ni,Co) alloys. The achieved maximal linear NTE reached Delta L/L similar to-23690 x 10(-6) = -121.5 x 10(-6) K-1) in a temperature interval as wide as -195 K (80-275 K) for Fe -doped MnNiGe alloys. This value was 3.3 times larger than that of the corresponding average crystallographical contribution and exceeded that of almost all NTE materials reported to date. Neutron powder diffraction and first principles calculations were carried out. The results revealed that Fe -doped MnNiGe showed incommensurate cone -spiral magnetic ordering, and the lattice distortion during the phase transition was more significant than that of MnCoGeln with linear ferromagnetic ordering. The larger lattice distortion favored the cleavage of the hexagonal phase along the c -axis. As a result, a texture effect along the (110) crystal plane occurred during the molding process, which greatly enhanced the amplitude of the isotropic in -plane linear NTE.
Title:
Symmetry transformation in Pd quasicrystals upon heating and hydrogenation
Authors:
Dubinko, V, Laptiev, D, Terentyev, D, Dmitriev, S, Irwin, K
Source:
COMP MATER SCI 177 10.1016/j.commatsci.2020.109582 MAY 2020
Abstract:
In this work, the structural transformation from a crystalline to quasicrystalline symmetry in palladium (Pd) and palladium-hydrogen (Pd-H) atomic clusters upon thermal annealing and hydrogenation has been addressed by means of atomistic simulations. A structural analysis of the clusters was performed during the heating up to the melting point to identify the temperature for the phase transformation. It has been demonstrated that nanometric pure Pd clusters transform from cuboctahedral to icosahedral structures under heating. This transformation is thermally activated process and the activation barrier depends on the cluster size. The activation energy of the cubo-ico symmetry transformation was measured using the variable heating rate method and was found to increase with the cluster size from 0.05 eV for 55 atomic cluster up to 0.66 eV for 147 atomic cluster. Hydrogenation of the nanometric Pd clusters yields to the modification of the transformation barrier in a nonmonotonic form. At low H concentration, the transformation barrier decreases, while by increasing H concentration above a certain threshold, the barrier grows again thus making a minimum around a specific hydrogen concentration. This behaviour was rationalized as a competition between two processes, namely: the structure symmetry breaking at low H concentrations and stabilization of cuboctahedral phase of the clusters at high H concentration. The obtained results provide an estimation of the temperature range at which the symmetry transformation should occur under thermal annealing with experimentally achievable heating rates.
Title:
Phase equilibria of low-Y side in Mg-Zn-Y system at 400 degrees C
Authors:
Liu, BS, Li, HX, Ren, YP, Jiang, M, Qin, GW
Source:
RARE METALS 39 (3):262 10.1007/s12598-018-1024-z MAR 2020
Abstract:
Phase equilibrium relations of the Mg-Zn-Y system in the low-Y side at 400 degrees C were investigated by alloy-equilibrated method, combined with thermal analysis. The results show that there is a liquid phase which could be in equilibrium with an alpha-Mg solid solution and an icosahedral quasicrystal I phase in the low-Y side of the Mg-Zn-Y system at 400 degrees C. The liquid phase region originates from the binary Mg-Zn system and extends to 0.4 at%Y in the Mg-Zn-Y system. Besides, the hexagonal structure H phase, fcc W phase and LPSO phase (X phase) are in equilibrium with alpha-Mg. With Y/Zn (atomic ratio, the same as follows) increasing, there exist four three-phase regions consisting of I + liquid + alpha-Mg, I + H + alpha-Mg, H + W + alpha-Mg and W + X + alpha-Mg, respectively, in the low-Y side of the isothermal section at 400 degrees C. The two-phase region alpha-Mg + I phase exists between I + H + alpha-Mg and I + liquid + alpha-Mg. In this two-phase region, the Y/Zn ratio is in the range of 0.14-0.17; and a three-phase region of alpha-Mg + I phase + H phase appears when Y/Zn ratio comes up to 0.17-0.27. Not I but W phase is in equilibrium with alpha-Mg, when Y/Zn ratio > 0.27. The system is in liquid-state phase equilibrium, when Y/Zn ratio < 0.14.
Update: 26-Mar-2020
Title:
Modulated Linear Tellurium Chains in Ba3ScTe5: Synthesis, Crystal Structure, Optical and Resistivity Studies, and Electronic Structure
Authors:
Ishtiyak, M, Panigrahi, G, Jana, S, Prakash, J, Mesbah, A, Malliakas, CD, Lebegue, S, Ibers, JA
Source:
INORG CHEM 59 (4):2434 10.1021/acs.inorgchem.9b03319 FEB 17 2020
Abstract:
A new ternary telluride, Ba3ScTe5, with a pseudo-one-dimensional structure, was synthesized at 1173 K by standard solid-state methods. A single-crystal X-ray diffraction study at 100(2) K shows the structure to be modulated. The structure of the subcell of Ba3ScTe5 crystallizes with two formula units in the hexagonal space group D-6h(3)-P6(3)/mcm with unit cell dimensions of a = b = 10.1190(5) angstrom and c = 6.8336(3) angstrom. The asymmetric unit of the subcell structure consists of four crystallographically independent sites: Ba1 (site symmetry: m2m), Sc1 (-3.m), Te1 (m2m), and Te2 (3.2). Its structure is made up of chains of (1)(infinity)[ScTe33-] that are separated by Ba2+ cations. The Sc atoms are bonded to six Te1 atoms that form a slightly distorted octahedral geometry. The structure of the subcell also contains linear infinite chains of Te2 with intermediate Te center dot center dot center dot Te interactions. The superstructure of Ba3ScTe5 is incommensurate and was solved in the hexagonal superspace group P-6(00 gamma)0 with a = 10.1188(3) angstrom and c = 6.8332(3) angstrom and a modulation vector of q = 0.3718(2)c*. The arrangement and coordination geometries of the atoms in the superstructure are very similar to those in the substructure. However, the main difference is that the infinite chains of Te atoms in the superstructure are distorted owing to the formation of long- and short-bonded pairs of Te atoms. The presence of these chains with intermediate Te center dot center dot center dot Te interactions makes assignment of the formal oxidation states arbitrary. The optical absorption study of a polycrystalline sample of Ba3ScTe5 that was synthesized by the stoichiometric reaction of elements at 1173 K reveals a direct band gap of 1.1(2) eV. The temperature-dependent resistivity study of polycrystalline Ba3ScTe5 shows semiconducting behavior corroborating the optical studies, while density functional theory calculations report a pseudo band gap of 1.3 eV.
Title:
Quasicrystal-related mosaics with periodic lattices interlaid with aperiodic tiles
ACTA CRYSTALLOGR A 76:137 10.1107/S2053273320000066 MAR 2020
Abstract:
Quasicrystals, which have long-range orientational order without translational symmetry, are incompatible with the theory of conventional crystals, which are characterized by periodic lattices and uniformly repeated unit cells. Reported here is a novel quasicrystal-related solid state observed in two Al-Cr-Fe-Si alloys, which can be described as a mosaic of aperiodically distributed unit tiles in translationally periodic structural blocks. This new type of material possesses the opposing features of both conventional crystals and quasicrystals, which might trigger wide interest in theory, experiments and the potential applications of this type of material.
Title:
The atomic structure of the Bergman-type icosahedral quasicrystal based on the Ammann-Kramer-Neri tiling
Authors:
Buganski, I, Wolny, J, Takakura, H
Source:
ACTA CRYSTALLOGR A 76:180 10.1107/S2053273319017339 MAR 2020
Abstract:
In this study, the atomic structure of the ternary icosahedral ZnMgTm quasicrystal (QC) is investigated by means of single-crystal X-ray diffraction. The structure is found to be a member of the Bergman QC family, frequently found in Zn-Mg-rare-earth systems. The ab initio structure solution was obtained by the use of the Superflip software. The infinite structure model was founded on the atomic decoration of two golden rhombohedra, with an edge length of 21.7 angstrom, constituting the Ammann-Kramer-Neri tiling. The refined structure converged well with the experimental diffraction diagram, with the crystallographic R factor equal to 9.8%. The Bergman clusters were found to be bonded by four possible linkages. Only two linkages, b and c, are detected in approximant crystals and are employed to model the icosahedral QCs in the cluster approach known for the CdYb Tsai-type QC. Additional short b and a linkages are found in this study. Short interatomic distances are not generated by those linkages due to the systematic absence of atoms and the formation of split atomic positions. The presence of four linkages allows the structure to be pictured as a complete covering by rhombic triacontahedral clusters and consequently there is no need to define the interstitial part of the structure (i.e. that outside the cluster). The 6D embedding of the solved structure is discussed for the final verification of the model.
Title:
Development of an Al-Mn-Si-Based Alloy with an Improved Quasicrystalline-Forming Ability
Authors:
Leskovar, B, Samardzija, Z, Koblar, M, Naglic, I, Markoli, B
This investigation deals with the influence of calcium, strontium and zinc on the formation of primary, metastable quasicrystals in Al-Mn-Si alloys as these can enhance the mechanical properties of alloys. The synthesized alloys were cast into a copper mould. The castings were characterized using standard tools for studying microstructure. The Al-Mn-Si-Zn-Ca-Sr alloy contained more of the primary icosahedral quasicrystalline (iQc) phase and less of the approximant alpha-Al-Mn-Si phase than the Al-Mn-Si alloy. The higher volume fraction of the primary iQc phase in the Al-Mn-Si-Zn-Ca-Sr alloy is explained by the reduction of the critical radius size for nucleation and by an increase in the number of nucleation sites for the iQc phase. Increased number of nucleation sites and higher volume fraction of the primary iQc are both related to a reduction of the surface tension for the Al-based melt caused by the presence of calcium and strontium. These two elements do not become incorporated into the iQc phase but have a large effect on the course of the solidification of the investigated alloys and their constitution at room temperature.
Title:
Effect of copper concentration on the structure and properties of Al-Cu-Fe and Al-Cu-Ni melts
Authors:
Kamaeva, LV, Ryltsev, RE, Suslov, AA, Chtchelkatchev, NM
Source:
J PHYS-CONDENS MAT 32 (22) 10.1088/1361-648X/ab73a6 MAY 20 2020
Abstract:
We address a relationship between properties of liquid and solid states by comparing structural characteristics and viscosity in Al-Cu-Fe and Al-Cu-Ni melts. The former system forms an equilibrium quasicrystalline phase but the latter does not. We show that the concentration behavior of the viscosity, melting temperature and characteristics of the chemical short-range order correlate with each other. The main structural differences between the melts are related to the peculiarities of their electronic structure, which is the same for liquid and solid states near the melting temperature.
Update: 19-Mar-2020
Title:
Transient three-dimensional structural dynamics in 1T -TaSe2
Authors:
Ji, SZ, Granas, O, Rossnagel, K, Weissenrieder, J
Source:
PHYS REV B 101 (9) 10.1103/PhysRevB.101.094303 MAR 5 2020
Abstract:
We report on thermal and optically driven transitions between the commensurate (C) and incommensurate (IC) charge-density wave (CDW) phases of 1T-TaSe2. Optical excitation results in suppression of the C-CDW on a subpicosecond timescale. The optically driven C to IC transition involves a short-lived (similar to 1 ps) unreconstructed phase. Nucleation of an IC phase stacking order is observed already at similar to 4 ps following photoexcitation. The short timescales involved in establishing the stacking order implies that the nucleation of the IC phase is influenced by the local geometry of the adjacent layers such that the stacking direction of the C phase determines the stacking direction of the IC phase. From this follows that the nucleation of the IC-CDW is inherently three dimensional (3D). We observe the activation of a coherent shear mode in the optically driven transitions to the transiently stabilized unreconstructed phase. The activation mechanism starts with a rapid lifting of the periodic lattice distortions (PLD) of the Ta sublattice which results in formation of local transient velocity disparities in the Se sublattice. The local differences in Se-phonon amplitudes result in noncompensated shear forces between the layers. This is an example of a multistep coherent launching mechanism. The energy of the optically excited electronic state dissipates energy into modes of the PLD through strong electron-phonon coupling. The rapid suppression of the PLD launches the third step, a coherent vibrational shear mode with low dissipation. The results highlight the importance in considering the 3D nature of the CDWs in the analysis of both structure and dynamics in transition-metal dichalcogenides.
Title:
Incommensurately Modulated Crystal Structure and Photoluminescence Properties of Eu2O3- and P2O5-Doped Ca2SiO4 Phosphor
Authors:
Nakano, H, Ando, S, Kamimoto, K, Hiramatsu, Y, Michiue, Y, Hirosaki, N, Fukuda, K
Source:
MATERIALS 13 (1) 10.3390/ma13010058 JAN 1 2020
Abstract:
We prepared four types of Eu2O3- and P2O5-doped Ca2SiO4 phosphors with different phase compositions but identical chemical composition, the chemical formula of which was (Ca1.950Eu0.0133+?(0.037))(Si0.940P0.060)O-4 (? denotes vacancies in Ca sites). One of the phosphors was composed exclusively of the incommensurate (IC) phase with superspace group Pnma(0 beta 0)00s and basic unit-cell dimensions of a = 0.68004(2) nm, b = 0.54481(2) nm, and c = 0.93956(3) nm (Z = 4). The crystal structure was made up of four types of beta-Ca2SiO4-related layers with an interlayer. The incommensurate modulation with wavelength of 4.110 x b was induced by the long-range stacking order of these layers. When increasing the relative amount of the IC-phase with respect to the coexisting beta-phase, the red light emission intensity, under excitation at 394 nm, steadily decreased to reach the minimum, at which the specimen was composed exclusively of the IC-phase. The coordination environments of Eu3+ ion in the crystal structures of beta- and IC-phases might be closely related to the photoluminescence intensities of the phosphors.
Title:
First-principles high pressure structure searching, longitudinal-transverse mode coupling and absence of simple cubic phase in sulfur
Authors:
Whaley-Baldwin, J, Needs, R
Source:
NEW J PHYS 22 (2) 10.1088/1367-2630/ab6068 FEB 2020
Abstract:
We use first-principles density functional theory to conduct an extensive structure search using the AIRSS package for elemental sulfur in the range 50-550 GPa. We then obtain the low-temperature phase diagram of sulfur in the same pressure range, including vibrational effects through the harmonic approximation. We do not find any structures lower in energy than those already reported in experiment, although the phase diagram below 100 GPa is found to be crowded with structures separated by only a few meV. We report the transition sequence I-41/acd -> P (1) over bar -> ICM -> C2/m -> R3 over bar m -> Im3 over bar m R3 over bar m
Title:
Complex magnetic order in the decorated spin-chain system Rb2Mn3(MoO4)(3)(OH)(2)
PHYS REV B 101 (6) 10.1103/PhysRevB.101.064423 FEB 24 2020
Abstract:
The macroscopic magnetic properties and microscopic magnetic structure of Rb2Mn3(MoO4)(3)(OH)(2) (space group Pnma) are investigated by magnetization, heat capacity, and single-crystal neutron diffraction measurements. The compound's crystal structure contains bond-alternating [Mn3O11](infinity) chains along the b axis, formed by isosceles triangles of Mn ions occupying two crystallographically nonequivalent sites (the Mn1 site on the base and Mn2 site on the vertex). These chains are only weakly linked to each other by nonmagnetic oxyanions. Both superconducting quantum interference device magnetometry and neutron diffraction experiments show two successive magnetic transitions as a function of temperature. On cooling, it transitions from a paramagnetic phase into an incommensurate phase below 4.5 K with a magnetic wave vector near k(1) = (0, 0.46, 0). An additional commensurate antiferromagnetically ordered component arises with k(2) = (0, 0, 0), forming a complex magnetic structure below 3.5 K with two different propagation vectors of different stars. On further cooling, the incommensurate wave vector undergoes a lock-in transition below 2.3 K. The experimental results suggest that the magnetic superspace group is Pnma.1'(0b0)s0ss for the single-k incommensurate phase and is Pn'ma(0b0)00s for the two-k magnetic phase. We propose a simplified magnetic structure model taking into account the major ordered contributions, where the commensurate k(2) defines the ordering of the c-axis component of the Mn1 magnetic moment, while the incommensurate k(1) describes the ordering of the ab-plane components of both Mn1 and Mn2 moments into elliptical cycloids.
Title:
Growth modes of quasicrystals involving intermediate phases and a multistep behavior studied by phase field crystal model
Authors:
Jiang, ZC, Quan, SL, Xu, N, He, LH, Ni, Y
Source:
PHYS REV MATER 4 (2) 10.1103/PhysRevMaterials.4.023403 FEB 26 2020
Abstract:
Understanding the microscopic kinetics of quasicrystal formation via nucleation and growth is crucial. Here, we report unusual pathways to nucleation and growth of dodecagonal quasicrystals via a phase field crystal model incorporating a two-length-scale potential. Under certain thermodynamic parameters, both quasicrystal growths via heterogeneous and homogeneous nucleation may be associated with a multistep behavior and the transient appearance of triangular and intermediate phases, different from classical nucleation pathways. The metastable intermediate phase spontaneously occurs to bridge the triangular phase and quasicrystal nuclei of different orientations to reduce the total free energy of the system. Decomposition of an undercooled fluid phase into quasicrystal phase shows a multistep pathway wherein the triangular phase and the intermediate phase may occur faster than the quasicrystal phase, when the growth rate of one length-scale ordering is significantly different from the other and the subsequent competing and coupling of both length scales are involved. The calculated structure factor, radial distribution function, and the aperiodic tiling structure of the intermediate phase explain why it appears during the quasicrystal formation.
Title:
Magnetic orders induced by RKKY interaction in Tsai-type quasicrystalline approximant Au-Al-Gd
Authors:
Miyazaki, H, Sugimoto, T, Morita, K, Tohyama, T
Source:
PHYS REV MATER 4 (2) 10.1103/PhysRevMaterials.4.024417 FEB 28 2020
Abstract:
Recent experimental study on the Tsai-type quasicrystalline approximant Au-Al-Gd, which is a crystal but has the same local structure as quasicrystals, has revealed the presence of magnetic orders and phase transitions with changing the Au/Al concentration. Motivated by the experiment, we theoretically investigate whether a successive change of magnetic orders occurs in a minimal magnetic model including the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction only. We find that the model induces multifarious magnetic orders depending on the Fermi wavenumber and gives a good starting point for understanding the experimental observation. In addition, we predict the presence of an undiscovered magnetic order called cuboc order at the large Fermi wavenumber region.
Title:
Origin of negative magnetization phenomena in (Tm1-xMnx)MnO3: A neutron diffraction study
Authors:
Donni, A, Pomjakushin, VY, Zhang, L, Yamaura, K, Belik, AA
Source:
PHYS REV B 101 (5) 10.1103/PhysRevB.101.054442 FEB 28 2020
Abstract:
(Tm1-xMnx)MnO3 solid solutions were synthesized at a high pressure of 6 GPa and a high temperature of about 1570-1670 K for 2 h for x = 0, 0.1, 0.2, and 0.3. Magnetic, dielectric, and neutron diffraction measurements revealed that the introduction ofmagnetic Mn2+ cations into the A site leads to an incommensurate spin structure for x = 0.1 and to a ferrimagnetic structure for x >= 0.2. Commensurate magnetic structures have a much larger correlation length (similar to 400 nm for x = 0, similar to 600 nm for x = 0.3) than the incommensurate magnetic structure (similar to 12 nm for x = 0.1). The presence of Tm3+ and Mn2+ (with different sizes) at the A site causes significant microstrain effects along the a direction which are absent for x = 0 and get stronger with increasing x. Magnetic ordering occurs at the Neel temperature T-N = 37 K (x = 0.1) and at the ferrimagnetic Curie temperatures T-C = 75 K (x = 0.2) and T-C = 104 K (x = 0.3). Ordering of magnetic Mn moments triggers short-range order (for x = 0.1) and long-range order (for x >= 0.2) of the Tm3+ cations at the same temperature-an unusual situation in perovskite materials with a simple GdFeO3-type Pnma structure. For x = 0.1, long-range IC magnetic order [with propagation vector k = (k(0), 0, 0) and k(0) approximate to 0.40] of Mn3+ and Mn4+ cations at the B site coexists with short-range order of Tm3+ and Mn2+ moments at the A site. Short-range order is induced at the Neel temperature T-N = 37 K, increases towards an additional specific heat anomaly at T = 4 K, and remains at lower temperature. The ferrimagnetic structure [with propagation vector k = (0, 0, 0)] consists of ferromagnetically ordered Mn3+ and Mn4+ cations at the B site which are coupled antiferromagnetically with ordered Mn2+ moments at the A site. Tm3+ moments adopt a zigzag magnetic structure which contains a macroscopic ferromagnetic moment that aligns with the direction of the ordered Mn2+ moments. Towards low temperature, the ordered Tm3+ moments strongly increase and overcome the saturated magnetic Mn moments at the B site, and this behavior results in the observation of magnetization reversal or negative magnetization phenomena with a compensation temperature of about 15 K at small magnetic fields in the x = 0.2 and 0.3 samples. This is a classical mechanism of the magnetization reversal effects for ferrimagnets.
Title:
Direct control of magnetic chirality in NdMn2O5 by external electric field
Authors:
Zobkalo, IA, Matveeva, AN, Sazonov, A, Barilo, SN, Shiryaev, SV, Pedersen, B, Hutanu, V
Source:
PHYS REV B 101 (6) 10.1103/PhysRevB.101.064425 FEB 25 2020
Abstract:
Detailed investigation of the incommensurate magnetic ordering in a single crystal of multiferroic NdMn2O5 has been performed using both nonpolarized and polarized neutron-diffraction techniques. Below T-N approximate to 30.5 K magnetic Bragg reflections corresponding to the nonchiral-type magnetic structure with propagation vector k(1) = (0.5 0 k(z1)) occurs. Below about 27 K a distorted magnetic modulation with a similar vector k(z2) occurs, which is attributed to the magnetization of the Nd3+ ions by the Mn sublattice. Strong temperature hysteresis in the occurrence of the incommensurate magnetic phases in NdMn2O5 was observed depending on the cooling or heating history of the sample. Below about 20 K the magnetic structure became of a chiral type. From spherical neutron polarimetry measurements, the resulting low-temperature magnetic structure k(z3) was approximated by the general elliptic helix. The parameters of the magnetic helixlike ellipticity and helical plane orientation in regard to the crystal structure were determined. A reorientation of the helix occurs at an intermediate temperature between 4 and 18 K. A difference between the population of right- and left-handed chiral domains of about 0.2 was observed in the as-grown crystal when cooling without an external electric field. The magnetic chiral ratio can be changed by the application of an external electric field of a few kV/cm, revealing strong magnetoelectric coupling. A linear dependence of the magnetic chirality on the applied electric field in NdMn2O5 was found. The results are discussed within the frame of the antisymmetric superexchange model for Dzyaloshinskii-Moriya interaction.
Title:
Analysis of 3D planar crack problems of one-dimensional hexagonal piezoelectric quasicrystals with thermal effect. part II: Numerical approach
Authors:
Li, Y, Qin, QH, Zhao, MH
Source:
INT J SOLIDS STRUCT 188:223 10.1016/j.ijsolstr.2019.10.020 APR 2020
Abstract:
Theoretical and numerical investigations on three-dimensional (3D) planar crack problems in one-dimensional (1D) hexagonal piezoelectric quasicrystals (QCs) with thermal effect are carried out systematically. Part II of the work aims to develop a general numerical approach to study 3D planar crack problems in 1D hexagonal piezoelectric QC media. Based on the theoretical formulations presented in Part I, a direct and effective method is proposed to derive the Green's functions for point extended displacement discontinuities (EDDs). These Green's functions are presented explicitly by a series of potential functions in a compact form. Using the superposition principle, the Green's functions for uniformly distributed EDDs over the crack elements are obtained. Related element solutions are used to construct the numerical approach, known as EDD boundary element method, for 1D hexagonal piezoelectric QCs. The proposed numerical method can be applicative to many complicated planar crack problems, such as multiple cracks, and cracks with non-uniform loadings, for 3D media composed of 1D hexagonal piezoelectric QCs with thermal effect. A comparison of the results obtained from the theoretical solutions given in Part I of the work with those obtained from the numerical method proposed here validates of the present investigation. (C) 2019 Elsevier Ltd. All rights reserved.
Title:
Analysis of 3D planar crack problems in one-dimensional hexagonal piezoelectric quasicrystals with thermal effect. part I: Theoretical formulations
Authors:
Li, Y, Qin, QH, Zhao, MH
Source:
INT J SOLIDS STRUCT 188:269 10.1016/j.ijsolstr.2019.10.019 APR 2020
Abstract:
Theoretical and numerical investigations on three-dimensional (3D) planar crack problems in one-dimensional (10) hexagonal piezoelectric quasicrystals (QCs) with thermal effect are performed systematically. Part I of this work derives a series of theoretical formulations that are then used to study the 3D planar crack problems in the QCs. The simple layer potential functions with the extended displacement discontinuities (EDDs) as the unknown variables and the general solution based on quasi-harmonic functions for the QCs under consideration are used to deduce the boundary equations that govern 3D planar crack problems. The hypersingular integral equation method is used to analyze the asymptotic singularities of the coupled thermal-electrical-phonon-phason fields near the crack edge. Expressions are then presented for the extended stress intensity factors (ESIFs) of a mixed model crack in terms of EDDs for arbitrarily-shaped cracks in the QCs, and the basic relationships between the energy release rate and the ESIFs are established. Closed-form solutions for some typical cracks, including an elliptical crack that is subjected to coupled electrical-phonon-phason loadings and a penny-shaped crack that is subjected to antisymmetric thermal loading, are determined via Fabrikant's analysis method. Additionally, both the physical quantities on the crack plane and the corresponding variables in the coupled thermal-electrical-phonon-phason field in the full space are given. The theoretical formulations derived in this paper provide a fundamental basis for development of the numerical approach proposed in Part II of our work, and can also serve as benchmarks for numerical solutions. (C) 2019 Elsevier Ltd. All rights reserved.
Title:
A NEW ANALYTICAL APPROACH FOR SOLVING EQUATIONS OF ELASTO-HYDRODYNAMICS IN QUASICRYSTALS
Authors:
Yakhno, V
Source:
J MECH MATER STRUCT 15 (1):135 10.2140/jomms.2020.15.135 JAN 2020
Abstract:
The dynamic equations for quasicrystals are written as time-dependent partial differential equations of the second order relative to phonon and phason displacements. In these equations phonons describe the dynamics of wave propagation and phasons describe diffusion process in quasicrystals. A new approach for deriving a solution (phonon and phason displacements) of the initial value problem is proposed. In this approach the Fourier transform with respect to 3D space variable of the given phonon, phason forces and initial displacements are assumed to be vector functions with components which have finite supports with respect to Fourier parameters for every fixed time variable. The equations for the Fourier images of displacements are reduced to a vector integral equation of the Volterra-type depending on Fourier parameters. The solution of the obtained vector integral equation is solved by successive approximations. Finally, phonon and phason displacements are derived by matrix transformations and the inverse Fourier transform to the solution of the vector integral equation.
Update: 12-Mar-2020
Title:
A15, sigma, and a Quasicrystal: Access to Complex Particle Packings via Bidisperse Diblock Copolymer Blends
ACS MACRO LETT 9 (2):197 10.1021/acsmacrolett.9b01026 FEB 2020
Abstract:
A renewed focus on the phase behavior of nominally single-component, compositionally asymmetric diblock copolymers has revealed a host of previously unanticipated Frank-Kasper (FK) and quasicrystalline phases. However, these periodic and aperiodic particle packings have thus far only been reported in low molecular weight, highly conformationally asymmetric diblock copolymers, leaving researchers with a relatively small library of polymers in which these phases can be studied. In this work, we report on a simple approach to access these morphologies: blending two diblock copolymers with the same corona block length and varied core block lengths. Compositionally symmetric and asymmetric polystyrene-b-1,4-polybutadiene (SB) diblock copolymers with constant corona block lengths were blended together and shown via small-angle X-ray scattering and transmission electron microscopy to order into the FK A15 and sigma phases, as well as a dodecagonal quasicrystal, providing a route to various particle packings in high molecular weight diblock copolymer melts.
Title:
Influence of isothermal structural transition on the magnetic properties of Cr doped Bi0.86Nd0.14FeO3 multiferroics
Authors:
Phong, PT, Salazar-Kuri, U, Van, HT, Khien, NV, Dang, NV, Tho, PT
Source:
J ALLOY COMPD 823 10.1016/j.jallcom.2020.153887 MAY 15 2020
Abstract:
In this work, we have studied the crystal structure, microstructure, magnetic properties, and the effect of isothermal structural transition on the magnetic properties of Bi0.86Nd0.14Fe1-xCrxO3 (0.02 <= x <= 0.1) ceramic compounds. The analysis of X-ray diffraction patterns reveals a mixture of the R3c rhombohedral and PbZrO3-type orthorhombic phases. With increasing Cr concentration, the PbZrO3-type phase percentage is gradually increased from 33% for x = 0.02-63% for x = 0.1 at the expense of the R3c phase. The magnetic properties of compounds are mainly attributed to the destruction of the long-range incommensurate cycloidal spin structure in the orthorhombic symmetry. The isothermal structural transition (IST) is observed at room temperature when samples are stored in laboratory conditions. A remarkable change of magnetic properties is observed, which is believed originated from the IST and spin frustration at the phase boundary. (C) 2020 Elsevier B.V. All rights reserved.
Title:
Magnetodielectricity induced by coexisting incommensurate conical magnetic structure and cluster glass-like states in polycrystalline BaFe10In2O19
Authors:
Gupta, S, Sathe, VG, Siruguri, V
Source:
J ALLOY COMPD 825 10.1016/j.jallcom.2020.154141 JUN 5 2020
Abstract:
Indium doped barium hexaferrite BaFe10In2O19 is shown to exhibit complex magnetic behaviour below 110 K wherein there is a coexistence of a long-range conical magnetic order along with a cluster spinglass-like state. Temperature dependent dc magnetization measurements reveal that the system exhibits an anomaly around 110 K. Temperature dependent neutron diffraction (ND) measurements measured in the Q-range 0.28-7.35 angstrom(-1) characterize this anomaly as a transition to a longitudinal conical magnetic state from the ferrimagnetic state at room temperature. Ac susceptibility and thermoremanent magnetization relaxation measurements also reveal the existence of cluster spin-glass like behavior below 110 K, indicating coexistence with conical magnetic order. The lattice parameters determined from the analysis of ND data show a sharp change at similar to 110 K which is also reflected in the anomalous changes seen in the temperature dependent Raman phonon modes around this temperature. This gives a clear indication of spin lattice coupling in the system. Finally, temperature and magnetic field dependent dielectric constant measurements reveal the occurrence of large magnetodielectric effect (MDE) near 110 K. This intriguing intrinsic magnetodielectric feature can be explained in terms of spin lattice coupling. (C) 2020 Elsevier B.V. All rights reserved.
Title:
Magnetic and structural transitions in EuAg4As2 studied using Eu-151 Mossbauer spectroscopy
Authors:
Ryan, DH, Bud'ko, SL, Hu, CW, Ni, N
Source:
AIP ADV 9 (12) 10.1063/1.5129682 DEC 1 2019
Abstract:
Eu-151 Mossbauer spectroscopy confirms that the europium in EuAg4As2 is fully divalent and that the spectrum at 5 K consists of a single, sharp magnetic pattern with a hyperfine field (B-hf) of 27.1(1) T and an isomer shift of -11.04(3) mm/s (relative to EuF3). The temperature dependence of the spectra shows that the ordering of the Eu moments proceeds via an incommensurate sine modulated structure starting at 15 K. The structure squares up below 9 K. A search for charge density or phonon softening signatures at the 120 K structural transition was not successful. (C) 2019 Author(s).
Title:
Effect of External Factors on Magnetism of Fluctuating Low-Dimensional Electron and Spin Correlations in Frustrated Manganites La1- ySmyMnO3+delta (y=0.85, 1.0)
Authors:
Bukhanko, FN, Bukhanko, AF
Source:
PHYS SOLID STATE+ 61 (12):2525 10.1134/S1063783419120084 DEC 2019
Abstract:
The effect of external factors on the temperature dependences of the magnetization of frustrated manganites La1 - ySmyMnO3 + delta (delta similar to 0.1, y = 0.85, 1.0) is studied. Two sharp peaks M(T) of different intensities detected in both samples at close temperatures T-1 and T-2 slightly higher than the critical temperature T-c of the coherent superconducting transition corresponds to the Lindhard divergence chi(L)(q(nest)) of the temperature dependence of the paramagnetic susceptibility of stripelike 1D electron/spin correlations modulated with wave vectors q(nest1) = 2k(F1) and q(nest2) = 2k(F2). The formation and evolution of magnetization features with increasing field are explained by the appearance of spatial modulation of electrical and magnetic properties in ab planes in the case of total nesting of electron-hole areas of the Fermi surface. This appears as two fragments of two fluctuating quasi-one-dimensional charge/spin density waves with wave vector q(1) || a and q(2) || b directions incommensurate with the lattice. It is assumed that the strong dependence of the magnetization of fluctuating 1D charge/spin density wave correlations on external influences is caused by the immediate vicinity of sample properties to the quantum critical point.
Title:
New Quasicrystal Approximant in the Sc-Pd System: From Topological Data Mining to the Bench
Authors:
Solokha, P, Eremin, RA, Leisegang, T, Proserpio, DM, Akhmetshina, TG, Gurskaya, A, Saccone, A, De Negri, S
Source:
CHEM MATER 32 (3):1064 10.1021/acs.chemmater.9b03767 FEB 11 2020
Abstract:
Intermetallics contribute significantly to our current demand for high-performance functional materials. However, understanding their chemistry is still an open and debated topic, especially for complex compounds such as approximants and quasicrystals. In this work, targeted topological data mining succeeded in (i) selecting all known Mackay-type approximants, (ii) uncovering the most important geometrical and chemical factors involved in their formation, and (iii) guiding the experimental work to obtain a new binary Sc-Pd 1/1 approximant for icosahedral quasicrystals containing the desired cluster. Single-crystal X-ray diffraction data analysis supplemented by electron density reconstruction using the maximum entropy method, showed fine structural peculiarities, that is, smeared electron densities in correspondence to some crystallographic sites. These characteristics have been studied through a comprehensive density functional theory modeling based on the combination of point defects such as vacancies and substitutions. It was confirmed that the structural disorder occurs in the shell enveloping the classical Mackay cluster, so that the real structure can be viewed as an assemblage of slightly different, locally ordered, four shell nanoclusters. Results obtained here open up broader perspectives for machine learning with the aim of designing novel materials in the fruitful field of quasicrystals and their approximants. This might become an alternative and/or complementary way to the electronic pseudogap tuning, often used before explorative synthesis.
Title:
Designing new quasicrystalline compositions in Al-based alloys
Authors:
Wolf, W, Bolfarini, C, Kiminami, CS, Botta, WJ
Source:
J ALLOY COMPD 823 10.1016/j.jallcom.2020.153765 MAY 15 2020
Abstract:
In the present work, we present and discuss the most important results that were obtained in the past few years related with the development of new quaternary quasicrystalline alloys and composites. We present the alloy design strategies that we have used and that led to the discovery of over 60 new quasicrystalline compositions, 2 new quasicrystal-forming systems and 1 new Al-matrix composite. New results are also presented and discussed here. Results of interest discussed in this work include: reassessment of quasicrystal and approximant phase formation on the Al-Co-Fe-Cr system; influence of Cr and Ni additions on the icosahedral Al-Cu-Fe quasicrystal stability; discovery of a decagonal Al-Ni-Co-Cr quasicrystal; composition range of formation of the decagonal Al-Cu-Fe-Cr quasicrystal using combinatorial strategies; fabrication of aluminum matrix composites reinforced with quasicrystals using conventional metallurgy fabrication methods. (C) 2020 Elsevier B.V. All rights reserved.
Title:
Synthesis of Quasicrysalline Powders and Coatings by Vacuum Arc Plasma Evaporation
PHYS SOLID STATE+ 61 (12):2547 10.1134/S1063783419120576 DEC 2019
Abstract:
Quasicrystalline coatings of the Al-Cu-Fe system were obtained by sputtering the cathode using a pulsed high-current low-pressure arc discharge followed by deposition on a hot (600 degrees C) and cooled substrate (25 degrees C). The surface morphology, chemical and phase composition of evaporation products in the powder form and the resulting coatings were studied. Powders and coatings were also annealed and the phase composition change was studied. It was found that during the evaporation process a significant change in chemical composition occurs in the form of loss of Al from the surface of particles and coating, that leads to a decrease in the content of the psi-phase. However, subsequent annealing and spraying onto a hot substrate leads to an increase in the psi-phase, as well as the density and hardness of the coatings.
Title:
Microstructure, mechanical properties and corrosion behavior of quasicrystal-reinforced Mg-Zn-Y alloy subjected to dual-frequency ultrasonic field
Authors:
Chen, XR, Ning, SC, Wang, A, Le, QC, Liao, QY, Jia, YH, Cheng, CL, Li, XQ, Atrens, A, Yu, FX
Source:
CORROS SCI 163 10.1016/j.corsci.2019.108289 FEB 2020
Abstract:
The dual-frequency ultrasonic field (DUF) was employed to refine the quasicrystal-reinforced Mg-Zn-Y alloy. Microstructural evolution, corrosion behavior and mechanical properties of as-cast Mg-Zn-Y alloys with and without DUF treatment were investigated. Results reveal that the DUF treatment can refine the alpha-Mg grain and the I-phase dramatically. Consequently, the corrosion resistance and the mechanical properties were improved. The increase of segregation of solute Zn element at grain boundaries and the morphology change of I-phase should respond for the promoting of corrosion resistance. The improvement of tensile strength can be ascribed to the tremendous grain refinement and strengthening effect of tiny I-phase quasicrystals.
Title:
Derivation of a solution of dynamic equations of motion for quasicrystals
Authors:
Yakhno, V
Source:
J ENG MATH 118 (1):63 10.1007/s10665-019-10014-w OCT 2019
Abstract:
The dynamic equations of motion in quasicrystals are written in terms of time-dependent partial differential equations of the second order relative to phonon and phason displacements. A method of derivation of a solution (phonon and phason displacements) of the initial value problem is proposed in this paper. In this method, images of the Fourier transform with respect to the 3D space variable of the given phonon, phason forces, and initial displacements are assumed to be vector functions with components which have finite supports for every fixed time variable. The Fourier images of displacements are computed by matrix transformations and solving ordinary differential equations, coefficients and non-homogeneous terms as well as initial data of which depend on 3D Fourier parameter. Finally, phonon and phason displacements are computed by the inverse Fourier transform to obtained Fourier image.
Update: 5-Mar-2020
Title:
The incommensurately modulated structures of low-temperature labradorite feldspars: a single-crystal X-ray and neutron diffraction study
Authors:
Jin, SY, Xu, HF, Wang, XP, Jacobs, R, Morgan, D
Source:
ACTA CRYSTALLOGR B 76:93 10.1107/S2052520619017128 FEB 2020
Abstract:
Labradorite feldspars of the plagioclase solid solution series have been known for their complicated subsolidus phase relations and enigmatic incommensurately modulated structures. Characterized by the irrationally indexed e-reflections in the diffraction pattern, e-labradorite shows the largest variation in the incommensurate ordering states among the e-plagioclase structures. The strongly ordered low-temperature e-labradorite is one of the last missing pieces of the e-plagioclase puzzle. Nine plutonic and metamorphic labradorite feldspar samples from Canada, Ukraine, Minnesota (USA), Tanzania and Greenland with compositions ranging from An(52.5) to An(68) were studied with single-crystal X-ray diffraction. Two crystals from Labrador, Canada, and Duluth, MN, USA, with wide enough twin lamellae were analyzed with single-crystal neutron diffraction. The incommensurately modulated structures of e-plagioclase are refined for the first time with neutron diffraction data, which confirmed that the T-O distance modulation in the low-temperature e-plagioclase results from the Al-Si ordering in the framework. Detailed configurations of the M site are also observed in the structures refined from neutron diffraction data, which were not possible to see with X-ray diffraction data. The relation between the q-vectors and the mole% An composition is revealed for the entire compositional range of e-plagioclase, from An(25) to An(75). The previously proposed two-trend relation depending on the cooling rate and phase transition path is confirmed. A new classification of e-plagioclase (e(alpha), e(beta) and e(gamma)) is proposed based on the q-vector of the structure, which makes it an independent character from the presence/absence of density modulation. New parameters are proposed to quantify the ordering states of these complicated aperiodic structures of e-plagioclases, such as the difference between < T(1)o-O > and < T(1)m-O > at phase t = 0.2 or the normalized intensity of the (071 (1) over bar) reflection.
Title:
On the puzzling case of sodium saccharinate 1.875-hydrate: structure description in (3+1)-dimensional superspace
Authors:
Rekis, T, Schonleber, A, van Smaalen, S
Source:
ACTA CRYSTALLOGR B 76:18 10.1107/S2052520619014938 FEB 2020
Abstract:
The structure of sodium saccharinate 1.875-hydrate is presented in three- and (3+1)-dimensional space. The present model is more accurate than previously published superstructures, due to an excellent data set collected up to a high resolution of 0.89 angstrom(-1). The present study confirms the unusual complexity of the structure comprising a very large primitive unit cell with Z' = 16. A much smaller degree of correlated disorder of parts of the unit cell is found than is present in the previously published models. As a result of pseudo-symmetry, the structure can be described in a higher-dimensional space. The X-ray diffraction data clearly indicate a (3+1)-dimensional periodic structure with stronger main reflections and weaker superstructure reflections. Furthermore, the structure is established as being commensurate. The structure description in superspace results in a four times smaller unit cell with an additional base centring of the lattice, resulting in an eightfold substructure (Z' = 2) of the 3D superstructure. Therefore, such a superspace approach is desirable to work out this high-Z' structure. The displacement and occupational modulation of the saccharinate anions have been studied, as well as their conformational variation along the fourth dimension.
Title:
Compositely modulated structures of phosphor materials SrxLi2+xAl2-xO4:Eu2+
Authors:
Ooishi, A, Michiue, Y, Funahashi, S, Takeda, T, Hirosaki, N
Source:
ACTA CRYSTALLOGR B 76:76 10.1107/S2052520619016391 FEB 2020
Abstract:
Composite crystals SrxLi2+xAl2-xO4:Eu2+ were synthesized and their structures were determined using single-crystal X-ray diffraction. The commensurate structure with a modulation wavevector q = 5c*/6 was analyzed in a conventional manner in 3D space, while a structure model in (3+1)-dimensional superspace was used for the other two crystals with modulation wavevectors slightly differing from 5c*/6. The superstructure of the commensurate phase was described using the space group P4/n and a common superspace group I4/m(00 gamma)00 was used for the (3+1)D structures of all three crystals. The whole structure of each crystal consists of two substructures. Basis vectors a and b are common, but c is different for the two substructures. The first substructure is a host framework constructed by (Li/Al)O-4 tetrahedra sharing edges. A linear connection of cavities is seen to be channel-like, in which Sr ions locate as guest cations forming the second substructure. The crystal of q = 5c*/6 contains five Sr ions per six cavities in a channel. Sr ions are distributed at seven sites, some of which are partially occupied. Statistical disorder of local structure models for the location of Sr ions in the channel was assumed to explain the results. Such a partially disordered character was also seen in the incommensurate phases and properly embodied by a (3+1)D model containing an atomic domain of the Sr ion with occupational modulation. Plots of the occupation factor, interatomic distances and the bond valence sum at each metal site as functions of t (= x(4) - q.r) are roughly identical in the three crystals, which are considered as members of the same series of composite crystals.
Title:
Field-induced magnetic incommensurability in multiferroic Ni3TeO6
Authors:
Lass, J, Andersen, CR, Leerberg, HK, Birkemose, S, Toth, S, Stuhr, U, Bartkowiak, M, Niedermayer, C, Lu, ZL, Toft-Petersen, R, Retuerto, M, Birk, JO, Lefmann, K
Source:
PHYS REV B 101 (5) 10.1103/PhysRevB.101.054415 FEB 10 2020
Abstract:
Using single-crystal neutron diffraction we show that the magnetic structure Ni3TeO6 at fields above 8.6 T along the c axis and low temperature changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector Q(C) = (0 0 1.5) to a conical spiral with propagation vector Q(IC )= (0 0 1.5 +/- delta), delta similar to 0.18, having a significant spin component in the (a, b) plane. We determine the phase diagram of this material in magnetic fields up to 10.5 T along c and show the phase transition between the low field and conical spiral phases is of first order by observing a discontinuous jump of the ordering vector. Q(IC) is found to drift both as a function of magnetic field and temperature. Preliminary inelastic neutron-scattering data reveal that the spinwave gap in zero field has minima exactly at Q(IC )and a gap of about 1.1 meV consisting with a crossover around 8.6 T. Further, a simple magnetic Hamiltonian accounting in broad terms for these is presented. Our findings confirm the exclusion of the inverse Dzyaloshinskii-Moriya interaction as a cause for the giant magnetoelectric due to symmetry arguments. In its place we advocate for the symmetric exchange striction as the origin of this effect.
Title:
Structure of icosahedral quasicrystals within the multiple-cell approach
Authors:
Madison, AE, Madison, PA
Source:
STRUCT CHEM 31 (1):485 10.1007/s11224-019-01430-w FEB 2020
Abstract:
The multiple-cell approach is discussed as an alternative to the higher-dimensional crystallographic description of icosahedral quasicrystals. Four types of quasi-unit cells fill the space without gaps and overlappings. Every cell in the whole tiling is decorated by specific atoms in a particular way and is associated with a triad: type, position, and orientation. The key features of the proposed approach are the subgroup/coset decomposition of icosahedral symmetry groups in accordance with the orbit-stabilizer theorem, a strict mathematical formalization of the substitution rules for all types of quasi-unit cells in the Socolar-Steinhardt tiling, formalization of the recursive inflation/deflation rules, and the eigenvalue-eigenvector analysis of corresponding substitution matrices. The similar approach can be applied to almost all types of quasicrystals.
Title:
Unusual charge density wave transition and absence of magnetic ordering in Er2Ir3Si5
Authors:
Ramakrishnan, S, Schonleber, A, Rekis, T, van Well, N, Noohinejad, L, van Smaalen, S, Tolkiehn, M, Paulmann, C, Bag, B, Thamizhavel, A, Pal, D, Ramakrishnan, S
Source:
PHYS REV B 101 (6) 10.1103/PhysRevB.101.060101 FEB 10 2020
Abstract:
The first-order charge density wave (CDW) phase transition of Er2Ir3Si5 is characterized by a crystal structure analysis, and electrical resistivity, magnetic susceptibility and specific heat measurements. The incommensurate CDW is accompanied by a strong lattice distortion, from which it is shown that the CDW resides on zigzag chains of iridium atoms. The CDW transition affects the magnitude of the local magnetic moments on Er3+, implying strong coupling between CDW and magnetism. This could account for the observation that magnetic order is suppressed down to at least 0.1 K in the high-quality single crystal presently studied. Any disorder in the crystallinity, as in ceramic material, broadens and suppresses the CDW transition, while magnetic order appears at 2.1 K.
Title:
Incommensurate crystal structure of PbHfO3
Authors:
Bosak, A, Svitlyk, V, Arakcheeva, A, Burkovsky, R, Diadkin, V, Roleder, K, Chernyshov, D
Source:
ACTA CRYSTALLOGR B 76:7 10.1107/S205252061901494X FEB 2020
Abstract:
Controversy in the description/identification of so-called intermediate phase(s) in PbHfO3, stable in the range similar to 420-480 K, has existed for a few decades. A synchrotron diffraction experiment on a partially detwinned crystal allowed the structure to be solved in the superspace group Imma(00 gamma)s00 (No. 74.2). In contrast to some previously published reports, in the pure compound only one distinct phase was observed between Pbam PbZrO3-like antiferroelectric and Pm3m paraelectric phases. The modulation vector depends only slightly on temperature. The major structure modulation is associated with the displacement of lead ions, which is accompanied by a smaller amplitude modulation for the surrounding O atoms and tilting of HfO6 octahedra. Tilting of the octahedra results in a doubling of the unit cell compared with the parent structure.
Title:
Magnetic phase diagram enriched by chemical substitution in a noncentrosymmetric helimagnet
Authors:
Sato, T, Araki, Y, Miyake, A, Nakao, A, Abe, N, Tokunaga, M, Kimura, S, Tokunaga, Y, Arima, T
Source:
PHYS REV B 101 (5) 10.1103/PhysRevB.101.054414 FEB 10 2020
Abstract:
We modify an antiferromagnetic-based helimagnet Ni2InSbO6 to a ferrimagnetic-based helimagnet by partial substitution of Cr for In. Upon applying a magnetic field, uncompensated moments bring about the collinear ferrimagnetic structure featured by the plateaulike magnetization and the disappearance of the magnetic superstructure. Furthermore, we have found an incommensurate magnetic phase distinct from the zero-field helix phase just below the Neel temperature as magnetic skyrmion crystal phases in some other noncentrosymmetric magnets. These findings provide a methodology to design non-coplanar magnetic states based on the material database.
Title:
Icosahedral quasicrystals and their cubic approximants in the Cd-Mg-RE (RE = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm) systems
Authors:
Labib, F, Fujita, N, Ohhashi, S, Tsai, AP
Source:
J ALLOY COMPD 822 10.1016/j.jallcom.2019.153541 MAY 5 2020
Abstract:
The formation and stability of icosahedral quasicrystals and their cubic approximants in the Cd-Mg-RE (RE = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm) systems are studied in detail, with a focus on the effects of Mg substitution as well as the dependence on temperature and RE species. The results show that the compositional region of the icosahedral phase in the ternary phase diagram shrinks gradually as the RE element becomes smaller in atomic size. This phenomenon is correlated with the geometric 'stability' of a so-called Tsai-type cluster described as a packing of spheres that represent the atomic radii of the constituent elements. Moreover, the icosahedral quasicrystals are found to be stabilized at lower RE concentrations (12-13.5 at.%) and temperatures (T < 773 K) compared to their cubic approximants, contrary to earlier findings for the Cd-Mg-Yb system. Single grains of the icosahedral, 2/1 and 1/1 cubic approximant phases with, respectively, rhombic triacontahedron, octahedron and rhombic dodecahedron morphologies ranging approximate to 1.5-2.5 mm in diameter are further synthesized using a self-flux method. It is confirmed that the bounding facets of the single grains are {111} and {110} for the 2/1 and 1/1 cubic approximants, respectively, through orientation analysis using electron backscatter diffraction. Finally, microstructures involving the Cd-Mg-RE icosahedral quasicrystal and Cd-Mg hexagonal solid solution are analysed to show parallel alignment of 2-fold rotational axes of the former with the 6-fold and 2-fold rotational axes of the latter. This orientation relationship is further discussed in terms of the coincidence of interplanar spacings between the two phases. (C) 2019 Elsevier B.V. All rights reserved.
Title:
Study of the phase diagram of the Kitaev-Hubbard chain
Authors:
Mahyaeh, I, Ardonne, E
Source:
PHYS REV B 101 (8) 10.1103/PhysRevB.101.085125 FEB 14 2020
Abstract:
We present a detailed study of the phase diagram of the Kitaev-Hubbard chain, that is the Kitaev chain in the presence of a nearest-neighbor density-density interaction, using both analytical techniques as well as DMRG. In the case of a moderate attractive interaction, the model has the same phases as the noninteracting chain, a trivial and a topological phase. For repulsive interactions, the phase diagram is more interesting. Apart from the previously observed topological, incommensurate, and charge density wave phases, we identify the "excited state charge density wave" phase. In this phase, the ground state resembles an excited state of an ordinary charge density phase, but is lower in energy due to the frustrated nature of the model. We find that the dynamical critical exponent takes the value z similar or equal to 1.8. Interestingly, this phase only appears for even system sizes, and is sensitive to the chemical potential on the edges of the chain. For the topological phase, we present an argument that excludes the presence of a strong zero mode for a large part of the topological phase. For the remaining region, we study the time dependence of the edge magnetization (using the bosonic incarnation of the model). These results further expand the region where a strong zero mode does not occur.
Title:
Unique growth mode observed in a Pb thin film on the threefold surface of an i-Ag-In-Yb quasicrystal
Authors:
Coates, S, Thorn, S, McGrath, R, Sharma, HR, Tsai, AP
Source:
PHYS REV MATER 4 (2) 10.1103/PhysRevMaterials.4.026003 FEB 11 2020
Abstract:
Novel epitaxial quasicrystalline films can be grown using the surfaces of intermetallic quasicrystals as templates. Here, we present a study of Pb adsorption on the threefold i-Ag-In-Yb surface, where Pb grows in a manner contrasting with conventional thin-film growth modes. Pb atoms are found to adsorb at sites over a range of heights, which are explained by bulk atomic positions left vacant by surface truncation, producing three-dimensional, isolated quasicrystalline Pb structures. This finding is contrasted with the growth of Pb on the more commonly used fivefold surface of the same quasicrystal, where smooth epitaxial layers result. We suggest that this unique structure originates due to the lower atomic density of the threefold surface, compared to the fivefold surface Similar atomic density can be found in lower symmetry planes of periodic systems, but these planes are often unstable and become facetted. This stable low-density quasicrystalline substrate provides a facile route to achieve this type of templated growth.
Update: 28-Feb-2020
Title:
Analysis of Incommensurate Magnetic Structures of Rare-Earth Intermetallides Tb3Ni and Ho7Rh3 Using the Magnetic Supersymmetry Group Formalism
Authors:
Gubkin, AF, Vaulin, AA, Tsutaoka, T, Baranov, NV
Source:
PHYS MET METALLOGR+ 120 (12):1152 10.1134/S0031918X19120068 DEC 2019
Abstract:
Complex incommensurate magnetic structures of Tb3Ni and Ho7Rh3 compounds have been analyzed using the magnetic superspace group approach. It has been established that the high-temperature incommensurate magnetic structure of Tb3Ni is described by the magnetic superspace group P112(1)/a1'(ab0)0ss and the incommensurate magnetic structure of Ho7Rh3 is described by the magnetic superspace group P6(3)1' (00g) hs. Using the example of the Tb3Ni and Ho7Rh3 compounds, it is shown that the magnetic superspace groups formalism is the most efficient approach when establishing and describing complex incommensurate magnetic structures.
Title:
Three-dimensional nonlocal buckling of composite nanoplates with coated one-dimensional quasicrystal in an elastic medium
Authors:
Guo, JH, Sun, TY, Pan, EN
Source:
INT J SOLIDS STRUCT 185:272 10.1016/j.ijsolstr.2019.08.033 MAR 1 2020
Abstract:
Based on the nonlocal theory, three-dimensional (3D) buckling of composite nanoplates with coated one-dimensional (1D) quasicrystal (QC) is analyzed. The nanoplate is embedded in an elastic medium and is under uniaxial or biaxial compression. All edges of the QC nanoplate are simply supported and its interaction with the surrounding medium is simulated by the Pasternak-type model. In terms of the extended displacement and traction vectors, the eigensystem is first derived from the basic equations of nonlocal QC materials. Then 3D analytical solutions of the critical buckling load under compression are derived by using the propagator matrix method and the continuity condition on the interfaces of the nanoplate. The influence of the thickness and length-to-width ratio of the nanoplate, Winkler stiffness and shear modulus of the elastic medium, coating thickness and nonlocal parameter on the critical buckling load is analyzed. For a sandwich nanoplate made of QC and soft metallic aluminium, our numerical results indicate that QC coatings could offer an interesting alternative to surface reinforcement of soft metallic materials in industrial applications. The present 3D buckling model could further serve as a benchmark for various thin-nanoplate theories and for numerical methods in multilayered QC nanoplate modeling with nonlocal effect. (C) 2019 Elsevier Ltd. All rights reserved.
Update: 20-Feb-2020
Title:
Incommensurately modulated crystal structure of flamite - natural analogue of alpha(H)'-Ca2SiO4
Authors:
Rashchenko, SV, Seryotkin, YV, Sokol, EV, Kokh, SN
Source:
ACTA CRYSTALLOGR B 75:1137 10.1107/S2052520619013623 DEC 2019
Abstract:
Crystal structures of unquenchable high-temperature polymorphs of Ca2SiO4, important in cement chemistry, have eluded single-crystal X-ray analysis. However, the problem may be addressed by studying chemically stabilized Ca2SiO4 polymorphs at ambient temperature. Here an incommensurately modulated crystal structure of flamite [Pnma(0 beta 0)00s, q = 0.2728 (2)b*, a = 6.8588 (2) angstrom, b = 5.4301 (2) angstrom, c = 9.4052 (3) angstrom] is described. It is a mineral analogue of orthorhombic alpha(H)'-Ca2SiO4 (stable between 1160 and 1425 degrees C), naturally stabilized by substitution with phosphorus. The incommensurate modulation results from wave-like displacement of cation sites accompanied by tilting of (Si,P)O-4 tetrahedra and variation of the Na/(Ca + Na + K) ratio along the modulation period. The studied sample from Hatrurim Basin (Negev Desert, Israel) with composition (Ca1.75K0.12Na0.12)(1.99)(Si0.74P0.26)(1.00)O-4 also demonstrates pseudomerohedral cyclic twinning around the a axis, which results from pseudohexagonal topology of the crystal structure and complicates the indexing of X-ray diffraction data.
Title:
The structures and phase transitions in 4-amino-pyridinium tetraaquabis(sulfato)iron(III), (C5H7N2)[Fe-III(H2O)(4)(SO4)(2)]
Authors:
Bednarchuk, TJ, Hornfeck, W, Kinzhybalo, V, Zhou, ZY, Dusek, M, Pietraszko, A
Source:
ACTA CRYSTALLOGR B 75:1144 10.1107/S2052520619013155 DEC 2019
Abstract:
The organic-inorganic hybrid compound 4-aminopyridinium tetraaquabis(sulfato)iron(III), (C5H7N2)[Fe-III(H2O)(4)(SO4)(2)] (4apFeS), was obtained by slow evaporation of the solvent at room temperature and characterized by single-crystal X-ray diffraction in the temperature range from 290 to 80 K. Differential scanning calorimetry revealed that the title compound undergoes a sequence of three reversible phase transitions, which has been verified by variable-temperature X-ray diffraction analysis during cooling-heating cycles over the temperature ranges 290-100-290 K. In the room-temperature phase (I), space group C2/c, oxygen atoms from the closest Fe-atom environment (octahedral) were disordered over two equivalent positions around a twofold axis. Two intermediate phases (II), (III) were solved and refined as incommensurately modulated structures, employing the superspace formalism applied to single-crystal X-ray diffraction data. Both structures can be described in the (3+1)-dimensional monoclinic X2/c(alpha,0,gamma)0s superspace group (where X is 1/2, 1/2, 0, 1/2) with modulation wavevectors q = (0.2943, 0, 0.5640) and q = (0.3366, 0, 0.5544) for phases (II) and (III), respectively. The completely ordered low-temperature phase (IV) was refined with the twinning model in the triclinic P (1) over bar space group, revealing the existence of two domains. The dynamics of the disordered anionic substructure in the 4apFeS crystal seems to play an essential role in the phase transition mechanisms. The discrete organic moieties were found to be fully ordered even at room temperature.
Title:
Highly mobile twin boundaries in seven-layer modulated Ni-Mn-Ga-Fe martensite
Authors:
Sozinov, A, Musiienko, D, Saren, A, Vertat, P, Straka, L, Heczko, O, Zeleny, M, Chulist, R, Ullakko, K
Source:
SCRIPTA MATER 178:62 10.1016/j.scriptamat.2019.10.042 MAR 2020
Abstract:
We report on low twinning stress of 0.2 MPa for type 1 twin boundaries with rational (101) twinning plane and 0.1 MPa for type 2 twin boundaries with rational [(1) over bar 01] twinning direction, in Ni2MnGa0.8Fe0.2 seven-layer modulated martensite exhibiting about 12% stress-induced strain. The studied samples were free of compound twins with (100) and (110) twinning planes. The martensite lattice is monoclinic and shows a long-period commensurate modulation with wave vector q=(2/7)g(110), where g(110) is the reciprocal lattice vector. The investigated alloy has high potential for practical applications due to the low twinning stress and the elevated Curie point T-C=400 K. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.
Update: 13-Feb-2020
Title:
MnSnTeO6: A Chiral Antiferromagnet Prepared by a Two-Step Topotactic Transformation
Authors:
Zvereva, E, Bukhteev, K, Evstigneeva, M, Komleva, E, Raganyan, G, Zakharov, K, Ovchenkov, Y, Kurbakov, A, Kuchugura, M, Senyshyn, A, Streltsov, S, Vasiliev, A, Nalbandyan, V
Source:
INORG CHEM 59 (2):1532 10.1021/acs.inorgchem.9b03423 JAN 20 2020
Abstract:
MnSnTeO6, a new chiral antiferromagnet, was prepared both by topotactic transformation of the metastable rosiaite-type polymorph and by direct synthesis from coprecipitated hydroxides. Its structure and its static and dynamic magnetic properties were studied comprehensively both experimentally (through X-ray and neutron powder diffraction, magnetization, specific heat, dielectric permittivity, and ESR techniques) and theoretically (by means of ab initio density functional theory (DFT) calculations within the spin-polarized generalized gradient approximation). MnSnTeO6 is isostructural with MnSb2O6 (space group P321) and does not show any structural transition between 3 and 300 K. The magnetic susceptibility and specific heat exhibit an antiferromagnetic ordering at T-N approximate to 9.8 K, which is confirmed by low-temperature neutron data. At the same time, the thermodynamic parameters demonstrate an additional anomaly on the temperature dependences of magnetic susceptibility chi(T), specific heat C-p(T) and dielectric permittivity epsilon(T) at T* approximate to 4.9 K, which is characterized by significant temperature hysteresis. Clear enhancement of the dielectric permittivity at T* is most likely to reflect the coupling of dielectric and magnetic subsystems leading to development of electric polarization. It was established that the ground state of MnSnTeO6 is stabilized by seven exchange parameters, and neutron diffraction revealed incommensurate magnetic structure with propagation vector k = (0, 0, 0.183) analogous to that of MnSb2O6. Ab initio DFT calculations demonstrate that the strongest exchange coupling occurs between planes along diagonals. All exchange parameters are antiferromagnetic and reveal moderate frustration.
Title:
Periodic and Aperiodic Tiling Patterns from a Tetrablock Terpolymer System of the A(1)BA(2)C Type
Authors:
Miyamori, Y, Suzuki, J, Takano, A, Matsushita, Y
Source:
ACS MACRO LETT 9 (1):32 10.1021/acsmacrolett.9b00861 JAN 2020
Abstract:
Two tetrablock terpolymers of the S1IS2P type, where S, I, and P denote polystyrene, polyisoprene, and poly(2-vinylpyridine), respectively, were prepared anionically. S1IS2P-1 (S-1/I/S-2/P = 0.35/0.16/0.34/0.15, four numbers being volume fractions of S-1, I, S-2, and P block chains) has a structure with double hexagonal cylinders, while S1IS2P-2 (S-1/I/S-2/P = 0.47/0.15/0.22/0.16) has an unusual double tetragonal structure. Moreover, 13 binary blends were prepared from these two parent polymers. Among them, five blends with alpha(= phi(S1)(phi(S2)) covering the range 1.50 <= alpha <= 1.86 were confirmed to have a 3.3.4.3.4 Archimedean tiling structure, in which their P domains adopt five satellite I domains, while four blends with alpha covering the range 1.37 <= alpha <= 1.48 were revealed to be a quasicrystalline tiling structure with dodecagonal symmetry.
Title:
Higher-Order Topological Insulators in Quasicrystals
Authors:
Chen, R, Chen, CZ, Gao, JH, Zhou, B, Xu, DH
Source:
PHYS REV LETT 124 (3) 10.1103/PhysRevLett.124.036803 JAN 22 2020
Abstract:
Current understanding of higher-order topological insulators (HOTIs) is based primarily on crystalline materials. Here, we propose that HOTIs can be realized in quasicrystals. Specifically, we show that two distinct types of second-order topological insulators (SOTIs) can be constructed on the quasicrystalline lattices (QLs) with different tiling patterns. One is derived by using a Wilson mass term to gap out the edge states of the quantum spin Hall insulator on QLs. The other is the quasicrystalline quadrupole insulator (QI) with a quantized quadrupole moment. We reveal some unusual features of the corner states (CSs) in the quasicrystalline SOTIs. We also show that the quasicrystalline QI can be simulated by a designed electrical circuit, where the CSs can be identified by measuring the impedance resonance peak. Our findings not only extend the concept of HOTIs into quasicrystals but also provide a feasible way to detect the topological property of quasicrystals in experiments.
Update: 6-Feb-2020
Title:
NpSe2: a Binary Chalcogenide Containing Modulated Selenide Chains and Ambiguous-Valent Metal
ANGEW CHEM INT EDIT 58 (45):16130 10.1002/anie.201910353 NOV 4 2019
Abstract:
A new binary compound, NpSe2, possesses metal-chalcogen and chalcogen-chalcogen interactions different from those reported for other metal dichalcogenides. Its structure is incommensurately modulated and features linear Se chains and valence-ambiguous Np cations.
Title:
Structural, magnetic, and spin dynamical properties of the polar antiferromagnets Ni3-xCoxTeO6 (x=1, 2)
Authors:
Skiadopoulou, S, Retuerto, M, Borodavka, F, Kadlec, C, Kadlec, F, Misek, M, Prokleska, J, Deng, Z, Tan, XY, Frank, C, Alonso, JA, Fernandez-Diaz, MT, Croft, M, Orlandi, F, Manuel, P, McCabe, E, Legut, D, Greenblatt, M, Kamba, S
Source:
PHYS REV B 101 (1) 10.1103/PhysRevB.101.014429 JAN 21 2020
Abstract:
We present results of a multimethod investigation of the polar antiferromagnets Ni2CoTeO6 and NiCo2TeO6, inspired by the colossal magnetoelectric effect present in Ni3TeO6. Both compounds crystalize in the same polar space group R3 as Ni3TeO6, preserving the crystal symmetry at least from room temperature down to 2 K. Ni2CoTeO6 and NiCo2TeO6 undergo antiferromagnetic phase transitions at T-N = 55 and 52 K, and spin-flop transitions at an external magnetic field of approximately 8 and 4 T, respectively. Both compounds present an incommensurate antiferromagnetic helical structure with spins lying in the ab plane, in contrast to the collinear one along the c axis in Ni3TeO6. Moreover, dielectric anomalies are observed at their antiferromagnetic phase transitions, suggesting a magnetoelectric behavior. Spin and lattice dynamics studies by a combination of infrared, Raman, and terahertz spectroscopies were performed. Below T-N, in both Ni2CoTeO6 and NiCo2TeO6, low-frequency spin excitations extremely sensitive to external magnetic field were observed. At least one of these magnons was simultaneously seen in Raman and THz spectra of both compounds, therefore we propose to assign them to electromagnons.
Update: 31-Jan-2020
Title:
Yu-Shiba-Rusinov States in the Charge-Density Modulated Superconductor NbSe2
Authors:
Liebhaber, E, Gonzalez, SA, Baba, R, Reecht, G, Heinrich, BW, Rohlf, S, Rossnagel, K, von Oppen, F, Franke, KJ
Source:
NANO LETT 20 (1):339 10.1021/acs.nanolett.9b03988 JAN 2020
Abstract:
NbSe2 is a remarkable superconductor in which charge-density order coexists with pairing correlations at low temperatures. Here, we study the interplay of magnetic adatoms and their Yu-Shiba-Rusinov (YSR) bound states with the charge density order. Exploiting the incommensurate nature of the charge-density wave (CDW), our measurements provide a thorough picture of how the CDW affects both the energies and the wave functions of the YSR states. Key features of the dependence of the YSR states on adsorption site relative to the CDW are explained by model calculations. Several properties make NbSe2 a promising substrate for realizing topological nanostructures. Our results will be important in designing such systems.
Update: 23-Jan-2020
Title:
Host Polytypism and Structural Modulation in Two-Dimensional Fe(NCS)(2)-Based Metal-Organic Frameworks: Can Spin-Crossover Transitions Be Predicted?
Authors:
De La Pinta, N, Klar, PB, Breczewski, T, Madariaga, G
Source:
CRYST GROWTH DES 20 (1):422 10.1021/acs.cgd.9b01348 JAN 2020
Abstract:
The properties and crystal structure of two new polymorphs of [Fe(tvp)(2)(NCS)(2)]center dot tvp [tvp = trans-(4,4'-vinylenedipyridine)] are investigated. Despite being unusual in metal organic frameworks (MOFs), one of them shows a commensurate modulation with q = (1 /4, 0, -1/4) at room temperature and an unmodulated triclinic structure at lower temperatures that requires a noticeable reconstruction of the H-bond schema. Magnetically, it consists of weakly interacting Fe-II zigzag chains. The second compound does not show any structural instability, and the distribution of the almost isolated magnetic ions is square planar. The refined structure models are compared applying fully the superspace approach and group theoretical tools. The structural differences are deeply discussed in terms of host-host and host-guest interactions. Unexpectedly, none of the polymorphs exhibit spin crossover, although they seem to fulfill the structural requirements derived from a thorough comparison with a large family of Fe(NCS)(2)-based MOFs. The aim of this study is to understand the mechanisms enabling or blocking the cooperativity needed for a spin crossover phase transition. They seem to be more related to steric effects than to a delicate balance between elastic and magnetic interactions. In particular, a low compactness of the high-spin state structure appears as a necessary condition for spin crossover to occur.
Title:
Aperiodic topological crystalline insulators
Authors:
Huang, HQ, Wu, YS, Liu, F
Source:
PHYS REV B 101 (4) 10.1103/PhysRevB.101.041103 JAN 6 2020
Abstract:
Topological crystalline insulators (TCIs) are usually described with topological protection imposed by the crystalline symmetry. In general, however, the existence of TCI states does not necessitate the periodicity of crystals as long as an essential lattice symmetry can be identified. Here we demonstrate the compatibility of TCIs with aperiodic systems, as exemplified by an octagonal quasicrystal. The aperiodic TCIs we proposed are attributed to a band inversion mechanism, which inverts states with the same parity but opposite eigenvalues of a specific symmetry (such as mirror reflection). The nontrivial topology is characterized by a nonzero integer "mirror Bott index." Moreover, we demonstrate that the topological edge states and quantized conductance of the aperiodic TCI, which are robust against disorder, can be effectively manipulated by external electric fields. Our findings not only provide a better understanding of electronic topology in relation to symmetry but also extend the experimental realization of topological states to much broader material categories beyond crystals.
Title:
Correlated Unique Variation of Electrical Resistivity to Crystallization Behavior of the Zr52.5Cu17.9Ni14.6Al10Ti5 Metallic Glass
Authors:
Zhang, X, Jiang, X, Huo, GR, Zhang, YX, Qiao, Y, Ye, F, Liu, BB
Source:
METALS-BASEL 9 (12) 10.3390/met9121298 DEC 2019
Abstract:
Due to the differences between the glass and crystalline phases, crystallization of metallic glass occurs with heat release, volume shrinkage, and electrical resistivity drastic changes. Electrical resistivity of the Zr52.5Cu17.9Ni14.6Al10Ti5 metallic glass during crystallization was investigated under both continuous heating and isothermal annealing. This amorphous alloy exhibits a continuous variation instead of sharp decline when reaches the onset crystallization temperature. This unique variation was found to be related to the formation of a few quasicrystalline phases. The slower phase transformation process of this metallic glass brings lots of grain boundaries, which results in increasing of resistivity at the last stage during isothermal annealing. These results imply that electrical resistivity measurement is a more intuitive approach to investigate structure evolution of metallic glasses.
Title:
Diffraction of atoms by a quasicrystal
Authors:
Eroshenko, YN
Source:
PHYS-USP+ 62 (5):526 2019
Update: 16-Jan-2020
Title:
Direct observation of polar nanodomains in the incommensurate phase of (K0.96Rb0.04)(2)ZnCl4 crystals using piezo force microscopy
Piezo force microscopy has been used to visualize nanoscaled surface charge density waves in a modulated ferroelectric system for the first time. Those structures are found in incommensurate systems close to lock-in transition consisting of an ordered sequence of commensurate nanodomains separated by discommensurations. In ferroelectrics, these domains are antiparallel polarized. As a model system, K2ZnCl4 crystals doped with Rb2ZnCl4 is used where the nanodomain structure can be stabilized at room temperature. Lock-in amplifier techniques allow the mapping of domain morphology and also its coarsening in aged crystals. Our results directly confirm earlier molecular dynamics simulations found in the literature.
Update: 9-Jan-2020
Title:
Frustrated and Allowed Structural Transitions: The Theory-Guided Discovery of the Modulated Structure of IrSi
Authors:
Warden, HEM, Fredrickson, DC
Source:
J AM CHEM SOC 141 (49):19424 10.1021/jacs.9b10473 DEC 11 2019
Abstract:
To the experienced molecular chemist, predicting the geometries and reactivities of a system is an exercise in balancing simple concepts such as sterics and electronics. In this Article, we illustrate how recent theoretical developments can give this combination of concepts a similar predictive power in intermetallic chemistry through the anticipation and discovery of structural complexity in the nominally MnP-type compound IrSi. Analysis of the bonding scheme and DFT-Chemical Pressure (CP) distribution of the reported MnP-type structure exposes issues pointing toward new structural behavior. The placement of the Fermi energy below an electronic pseudogap indicates that this structure is electron-poor, an observation that can be traced via the 18-n rule to the structure's Ir-Ir connectivity. In parallel with this, the structure's CP scheme highlights facile paths of atomic motion that could enable a structural response to this electronic deficiency. Combined, these analyses suggest that IrSi may adopt a more complex structure than previously recognized. Through synthesis and detailed structural investigation of this phase, we confirm this prediction: single-crystal X-ray diffraction reveals an incommensurately modulated structure with the (3+1)D superspace group P2(1)/n(0 beta gamma)00 and q approximate to -0.22b* + 0.29c*. The structural modulations increase the average number of Ir-Ir bonds to nearly match the 18-n expectations of the phase through Ir-Ir trimerization along negative CPs with the incommensurability arising from the difficulty of contracting and stretching the Ir-Ir contacts in a regular pattern without expanding the structure along directions of negative Si-Si CP. Integrating these results with prior analyses of related systems points to a simple guideline for materials design, the Frustrated and Allowed Structural Transitions (FAST) principle: the ease with which competing structural phenomena can be experimentally realized is governed by the degree to which they are supported by the coordination of the atomic packing and electronic factors.
Title:
Effect of (Cu/Fe)O-5 bipyramid size and separation on magnetic and dielectric properties of rare earth layered perovskite LaBaCuFeO5 and LuBaCuFeO5
Authors:
Lal, S, Yadav, CS, Mukherjee, K
Source:
J APPL PHYS 126 (14) 10.1063/1.5096611 OCT 14 2019
Abstract:
We report structural, magnetic, and dielectric properties of layered perovskite materials LnBaCuFeO(5) (Ln = La and Lu). LaBaCuFeO5 shows magnetic cluster glass behavior below 60 K owing to the competing ferromagnetic and antiferromagnetic exchange interactions. Glassy dynamics of electric dipoles has also been observed in the vicinity of the magnetic glass transition temperature. The presence of significant coupling between spin and polar degrees of freedom results in the multiglass feature in LaBaCuFeO5. The LuBaCuFeO5 compound undergoes YBaCuFeO5 like commensurate to incommensurate antiferromagnetic transition at 175 K. Large magnetic irreversibility below 17 K in this compound suggests the presence of strong spin anisotropy. In addition, in this compound, the interaction between the dipoles is not strong enough, which results in the absence of glassy dynamics of electric dipoles. The contrasting behavior of two compounds is possibly due to variations in the ferromagnetic and antiferromagnetic interactions along the c-axis, which is the manifestation of structural modification arising out of the difference in the ionic radii of La and Lu. Published under license by AIP Publishing.
Title:
Rich diversity of crystallographic phase formation in 2D RexMo1-xS2 (x < 0.5) alloy
Authors:
Sharona, H, Vishal, B, Bhat, U, Paul, A, Mukherjee, A, Sarma, SC, Peter, SC, Datta, R
Source:
J APPL PHYS 126 (22) 10.1063/1.5121363 DEC 14 2019
Abstract:
We report on the observation of the rich variety of crystallographic phase formation in a RexMo1-xS2 alloy for x < 0.5. For x < 0.23, no low dimensional superstructural modulation is observed and inter cation hybridization remains discrete, forming dimers to tetramers with increasing Re concentration. For x > 0.23, superstructural modulation is observed. Depending on the Re concentrations (x = 0.23, 0.32, 0.38, and 0.45) and its distributions, various types of cation hybridization result in the rich variety of low dimensional superstructural modulation as directly revealed by high resolution transmission electron microscopy. These layered alloy systems may be useful for various energy and novel device applications. Published under license by AIP Publishing.
Title:
Phase selection and microstructure of slowly solidified Al-Cu-Fe alloys
Authors:
Kamaeva, LV, Sterkhova, IV, Lad'yanov, VI, Ryltsev, RE, Chtchelkatchev, NM
Source:
J CRYST GROWTH 531 10.1016/j.jcrysgro.2019.125318 FEB 1 2020
Abstract:
The search for effective methods to fabricate bulk single-phase quasicrystalline Al-Cu-Fe alloys is currently an important task. Crucial to solving this problem is to understand the mechanisms of phase formation in this system. Here we study the crystallization sequence during solidification as well as the conditions of solid phase formation in slowly solidified Al-Cu-Fe alloys in a wide range of compositions. We have also constructed concentration dependencies of undercoolability by differential thermal analysis method. These experimental results are compared with data on chemical short-range order in the liquid state determined from ab initio molecular dynamic simulations. We observe that the main features of interatomic interaction in the Al-Cu-Fe alloys are similar for both liquid and solid states and they change in the vicinity of i-phase composition. In the concentration region, where the i-phase is formed from the melt, both the undercoolability and the crystallization character depend on the temperature of the melts before cooling.
Title:
New experimental studies on the phase diagram of the Al-Cu-Fe quasicrystal-forming system
MATER DESIGN 185 10.1016/j.matdes.2019.108186 JAN 5 2020
Abstract:
Phase diagrams of quasicrystal-forming alloy systems, such as Al-Cu-Fe, are essential to the continued development and fundamental understanding of quasicrystalline alloys, especially for the synthesis of single quasicrystals. Isothermal sections of the Al-Cu-Fe system at 700, 800, and 1000 degrees C were constructed over the whole composition range by investigations of three Cu/Fe/Al5Fe2 diffusion triples and forty-seven equilibrated alloys using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). Based on experimental measurements and reasonable assumptions, eleven, eight, and six three-phase equilibria were obtained at 700, 800, and 1000 degrees C, respectively. The existence of the ternary I-Al6Cu2Fe icosahedral quasicrystalline phase was confirmed at both 700 and 800 degrees C, and the ternary omega-Al7Cu2Fe phase was only found at 700 degrees C. The stable composition range of the I phase was measured to be Al67-56Cu24-31Fe9-13 and Al66-58Cu23-28Fe11-14 at 700 and 800 degrees C, respectively. A wide continuous solution region formed among the alpha-Fe, beta-AlFe, and Cu3Al phases at 800 and 1000 degrees C. A sufficiently large amount of new experimental phase equilibrium data was collected for the Al-Cu-Fe system that will help improve future thermodynamic description of this ternary system within the CALPHAD (CALculation of PHAse Diagrams) framework for designing Al-Cu-Fe-based quasicrystals. (C) 2019 The Authors. Published by Elsevier Ltd.
Update: 3-Jan-2020
Title:
Dodecagonal bilayer graphene quasicrystal and its approximants
Authors:
Yu, GD, Wu, ZW, Zhan, Z, Katsnelson, MI, Yuan, SJ
Source:
NPJ COMPUT MATER 5 10.1038/s41524-019-0258-0 DEC 12 2019
Abstract:
Dodecagonal bilayer graphene quasicrystal has 12-fold rotational order but lacks translational symmetry which prevents the application of band theory. In this paper, we study the electronic and optical properties of graphene quasicrystal with large-scale tight-binding calculations involving more than ten million atoms. We propose a series of periodic approximants which reproduce accurately the properties of quasicrystal within a finite unit cell. By utilizing the band-unfolding method on the smallest approximant with only 2702 atoms, the effective band structure of graphene quasicrystal is derived. The features, such as the emergence of new Dirac points (especially the mirrored ones), the band gap at M point and the Fermi velocity are all in agreement with recent experiments. The properties of quasicrystal states are identified in the Landau level spectrum and optical excitations. Importantly, our results show that the lattice mismatch is the dominant factor determining the accuracy of layered approximants. The proposed approximants can be used directly for other layered materials in honeycomb lattice, and the design principles can be applied for any quasi-periodic incommensurate structures.
Title:
Ultrasmall Moment Incommensurate Spin Density Wave Order Masking a Ferromagnetic Quantum Critical Point in NbFe2
Authors:
Niklowitz, PG, Hirschberger, M, Lucas, M, Cermak, P, Schneidewind, A, Faulhaber, E, Mignot, JM, Duncan, WJ, Neubauer, A, Pfleiderer, C, Grosche, FM
Source:
PHYS REV LETT 123 (24) 10.1103/PhysRevLett.123.247203 DEC 11 2019
Abstract:
In the metallic magnet Nb1-yFe2+y, the low temperature threshold of ferromagnetism can be investigated by varying the Fe excess y within a narrow homogeneity range. We use elastic neutron scattering to track the evolution of magnetic order from Fe-rich, ferromagnetic Nb0.981Fe2.019 to approximately stoichiometric NbFe2, in which we can, for the first time, characterize a long-wavelength spin density wave state burying a ferromagnetic quantum critical point. The associated ordering wave vector q(SDW) = (0, 0, l(SDW)) is found to depend significantly on y and T, staying finite but decreasing as the ferromagnetic state is approached. The phase diagram follows a two-order-parameter Landau theory, for which all of the coefficients can now be determined. Our findings suggest that the emergence of spin density wave order cannot be attributed to band structure effects alone. They indicate a common microscopic origin of both types of magnetic order and provide strong constraints on related theoretical scenarios based on, e.g., quantum order by disorder.
Title:
Origin of hour-glass magnetic dispersion in underdoped cuprate superconductors
Authors:
Kharkov, YA, Sushkov, OP
Source:
PHYS REV B 100 (22) 10.1103/PhysRevB.100.224510 DEC 13 2019
Abstract:
In this work we explain the hour-glass magnetic dispersion in underdoped cuprates. The dispersion arises due to the Lifshitz-type magnetic criticality. Superconductivity also plays a role, but the role is secondary. We list six major experimental observations related to the hour glass and explain all of them. The theory provides a unified picture of the evolution of magnetic excitations in various cuprate families, including "hour-glass" and "wine-glass" dispersions and an emergent static incommensurate order. We propose the Lifshitz spin-liquid "fingerprint" sum rule, and show that the latest data confirm the validity of the sum rule.
Title:
Dynamics in two-dimensional glassy systems of crowded Penrose kites
PHYS REV MATER 3 (12) 10.1103/PhysRevMaterials.3.125603 DEC 16 2019
Abstract:
We investigate the translational and rotational relaxation dynamics of a crowded two-dimensional system of monodisperse Penrose kites, in which crystallization, quasicrystallization and nematic ordering are suppressed, from low to high area fractions along the metastable ergodic fluid branch. First, we demonstrate a decoupling between both the translational and the rotational diffusion coefficients and the relaxation time: the diffusivities are not inversely proportional to the relaxation time, neither in the low-density normal liquid regime nor in the high-density supercooled regime. Our simulations reveal that this inverse proportionality breaks in the normal liquid regime due to the Mermin-Wagner long-wavelength fluctuations and in the supercooled regime due to the dynamical heterogeneities. We then show that dynamical heterogeneities are mainly spatial for translational degrees of freedom and temporal for rotational ones, there is no correlation between the particles with largest translational and rotational displacements, and different dynamical length scales characterize the translational and the rotational motion. Hence, despite the translational and the rotational glass-transition densities coincide, according to a mode-coupling fit, translations and rotations appear to decorrelate via different dynamical processes.
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