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
Authors:
He, ZB, Shen, YH, Ma, HK, Sun, JL, Ma, XL, Li, H, Steurer, W
Source:
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
Source:
JOM-US 72 (4):1533 10.1007/s11837-019-03702-6 APR 2020
Abstract:
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)
Authors:
Liu, YH, Sanjeewa, LD, Garlea, VO, Pellizzeri, TMS, Kolis, JW, Sefat, AS
Source:
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
Authors:
Lindsay, AP, Lewis, RM, Lee, B, Peterson, AJ, Lodge, TP, Bates, FS
Source:
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
Authors:
Ushakov, AV, Karpov, IV, Fedorov, LY, Shaikhadinov, AA, Demin, VG, Demchenko, AI, Goncharova, EA, Zeer, GM
Source:
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
Authors:
Jin, GB, Malliakas, CD, Hu, YJ, Booth, CH, Ibers, JA, Sokaras, D, Weng, TC, Skanthakumar, S, Soderholm, L
Source:
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
Authors:
Kofahl, C, Guthoff, F, Eckold, G
Source:
FERROELECTRICS 540 (1):10 10.1080/00150193.2019.1611115 2019
Abstract:
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
Authors:
Zhu, LL, Soto-Medina, S, Cuadrado-Castillo, W, Hennig, RG, Manuel, MV
Source:
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
Authors:
Li, YW, Li, ZQ, Hou, ZL, Mason, TG, Zhao, K, Sun, ZY, Ciamarra, MP
Source:
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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