CsCl-type Structure Research Articles

Emergence of topological phase and non-trivial surface states in rare-earth semimetal GdSb with pressure

Abstract We study the evolution of band topology under external pressure in rare-earth gadolinium mono-antimonide (GdSb) using first-principles calculations. This material crystallizes in a rocksalt-type structure and shows a structural phase transition (SPT) to a CsCl-type structure at 26.1 GPa. The phonon dispersions are analyzed to ascertain the dynamical stability of this material. We use hybrid density functional theory with the inclusion of spin–orbit coupling to investigate the structural, electronic, and topological phase transitions (TPTs). At ambient pressure, GdSb shows a topologically trivial state which is in agreement with existing experimental reports. The first TPT is observed at 6 GPa of hydrostatic pressure (at the high symmetry X-point) which is verified with the help of single-band inversion and surface state analysis along the (001) plane. The non-zero value of the first Z2 topological invariant and the presence of the Dirac cone also confirm the topological phase of this material. A further increase in pressure to 12 GPa results in two band inversions at Γ- as well as X-points, which corresponds to the trivial nature of GdSb. The same is also verified with (0; 000) values of Z2 topological invariants and a pair of Dirac cones in surface states. It is noted that the crystal symmetries are preserved throughout the study and the TPT values are much lower than the SPT pressure, i.e. 26.1 GPa.

Appearance of topological phase in YAs semimetal under hydrostatic pressure and epitaxial strain

By means of hybrid density functional theory, we present the evolution of the topological phase in rare earth monopnictide YAs with hydrostatic pressure and epitaxial strain. This material exists in NaCl-type structure at ambient conditions and shows structural phase transition into CsCl-type structure at 56.54 GPa hydrostatic pressure. The epitaxial strain reduces the structure into a compressed tetragonal-type. The thermodynamical and dynamical stability of the material is established with the calculation of enthalpy and phonon band structure within structural phase transition, respectively. The topologically trivial phase of the material is observed at ambient pressure in agreement with previous reports. This material shows topological phase transition at 24.8 GPa applied hydrostatic pressure and 10% epitaxial strain. The band inversion at the X-point between d-Y and p-As orbitals is verified with the help of the product of parity analysis of all the filled bands. The presence of the Dirac cone in the (001) plane and the existence of topologically non-trivial states at M‾-point in the Fermi arc contour also established our claim. The Z2 indices are calculated with the help of the product of parities and a change in Z2 indices from (0; 000) to (1; 000) is also verified with the evolution of Wannier change centers under the conditions of applied hydrostatic pressure and epitaxial strain. The time reversal and inversion symmetries are preserved throughout the study and the topological phase transition at 24.8 GPa is much lower than the structural phase transition pressure i.e., 56.54 GPa.

Phase analysis of near equiatomic bulk FeRh alloys: X-ray versus neutron diffraction and magnetization measurements

The information obtained by X-ray diffraction (XRD) by impinging the X-ray beam onto the flat surface of bulk Fe–Rh alloys is contrasted with that provided by the thermomagnetic analysis curves measured under a magnetic field of 5 mT and 2 T, and the neutron diffraction (ND) patterns. The experiments were performed on slices cut from bulk Fe50Rh50 and Fe49Rh51 alloys prepared by induction melting followed by 48 h of thermal annealing at 1273 K. Whereas ND patterns and magnetization curves, directly and indirectly point out that the samples are single-phase with a CsCl-type crystal structure undergoing a magnetoelastic transition that changes the magnetic structure from antiferromagnetic to ferromagnetic on heating, multiple phases are identified in the XRD patterns, which modify upon polishing or etching the surface with a mixture of hydrochloric (HCl) and nitric acid (HNO3) at a volume ratio of 90:10 from 30 to 210 min. The limitation of XRD for accurately characterizing the phase constitution within the bulk of Fe–Rh alloys is underlined.

Organometallic Ionic Plastic Crystals Incorporating Cationic Half-Sandwich Complexes.

Ionic plastic crystals (IPCs), characterized by nearly spherical molecular ions, exhibit remarkable solid-state characteristics including high ionic conductivity. However, most IPCs are organic onium salts. Incorporating organometallic half-sandwich complexes into IPCs is challenging owing to their low-symmetry structures. This paper introduces a novel series of IPCs composed of salts derived from half-sandwich organometallic complexes. We synthesized five salts of [Ru(Cp)(tmeda)(CO)]X (tmeda = N,N,N',N'-tetramethyl-1,2-ethanediamine, X = anion) with different anions and examined their phase behavior, crystal structures, and molecular motion in the solid-state. Salts featuring the CPFSA (= 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide), B(CN)4-, and FSA- (= (FSO2)2N-) anions underwent phase transitions to an IPC phase with a CsCl-type structure in the temperature range of 327-364 K. Employing smaller anions led to an increase in the transition temperature. In each salt, the coordination number, representing the number of anions surrounding one cation, remained eight in IPC and low-temperature phases. However, salts containing smaller anions (CF3BF3- and PF6-) displayed a rotator phase rather than the IPC phase. In these cases, the coordination numbers were six at low temperatures.

In search of coloured alloys resembling gold: Structure and optical properties of LiCu2Ga1−xAlx and Li(Cu1–yNiy)2Ga

The Li-containing solid solutions LiCu2Ga1–xAlx and Li(Cu1–yNiy)2Ga were prepared by induction heating. LiCu2Ga1–xAlx retains the Cu2MnAl-type (full Heusler) structure of LiCu2Ga for x = 0–0.7 and then adopts the CsCl-type structure for x = 0.9–1.0. The solid solution Li(Cu1–yNiy)2Ga forms for y = 0–0.5 with the Cu2MnAl-type structure. Solid-state 7Li NMR spectra for LiCu2Ga1–xAlx suggest that Li atoms occupy unique sites, but there is some uncertainty about the intermediate members of this solid solution. These compounds show high reflectivity in the yellow region and an absorption edge caused by interband transitions from filled Cu 3d states to empty bands. The yellow colour of LiCu2Ga gradually changes to red as Ga is substituted by Al because the absorption edge shifts to lower energy, and it becomes lighter yellow as Cu is substituted by Ni because the relative reflectivity decreases. For low levels of substitution by Al (x = 0.1) or Ni (y = 0.1), the chromaticity coordinates are close to those of elemental gold.

Nature of the bond in intermetallic compounds

By studying 398 binary compounds crystallizing in five different crystal structures, it is shown that atomic radii of the elements RA and RB change to reach a constant radius ratio rA/rB(≡ϒ) in a cubic system. In systems of lower symmetry, a range of ϒ values, each corresponding to a certain axial ratio, is possible. Accurate charge values on atoms in solids are derived from the experimental internal radii rA and rB, using Shannon’s compilation of radii versus charge states for elements. It is found, for the first time, that charge transfer and radii change reverse their direction at the ϒ point. Electronegativity difference relative to its value at the ϒ point is recognized to correctly predict the charge transfer and radii changes with RA/RB. The deduced electronic structure of solids, based on spherical ions carrying partial charges, confirms that the metallic atom-pair bond is a resonating polar covalent bond. Identification of the number of electrons that contribute to cohesion and electrical conduction individually, allows successful prediction of electrical conductivity variation for transition metals and p-elements in the periodic table. From a knowledge of accurate valence of elements and of charge on the atoms, variation in the 197Au isomer shift of the gold compounds of CsCl type structure is predicted. The charges on atoms computed using the Quantum Theory of Atoms in Molecules (QTAIM) technique agree with those obtained in our work for compounds belonging to two of the structure types. For compounds belonging to the other three, the charge values obtained by the two methods do not agree. The disagreement correlates to not reckoning the role of the ϒ point. The effect of ϒ point is so fundamental and universal that most physical properties obtained by the secondary processing of primary DFT data, can be influenced by it.

Compressibility Studies of Copper Selenides Obtained by Cation Exchange Reaction Revealing the New CsCl Phase.

In this study, we conducted a high-pressure investigation of Cu2-xSe nanostructures with pyramid- and plate-like morphologies, created through cation exchange from zinc-blende CdSe nanocrystals and wurtzite CdSe nanoplatelets respectively. Using a diamond anvil cell setup at the APS synchrotron, we observed the phase transitions in the Cu2-xSe nanostructures up to 40 GPa, identifying a novel CsCl-type lattice with Pm3̅m symmetry above 4 GPa. This CsCl-type structure, previously unreported in copper selenides, was partially retained after decompression. Our results indicate that the initial crystalline structure of CdSe does not affect the stability of Cu2-xSe nanostructures formed via cation exchange. Both morphologies of Cu2-xSe sintered under compression, potentially contributing to the stabilization of the high-pressure phase through interfacial defects. These findings are significant for discovering new phases with potential applications in future technologies.

Pressure-induced superconductivity and robust Tc against external pressure in (Ge,Sn,Pb)Te

The robustness of superconducting transition temperature (Tc) against external pressure in medium entropy alloy (MEA) and high entropy alloy (HEA) -type compounds has attracted significant interest with regards to the realization of stable superconducting applications. In this study, we have synthesized Ge1/3Sn1/3Pb1/3Te belonging to MEA-type MTe, where the M site comprises only isovalent group-14 elements, to depict a critical factor for the robustness of Tc. High-pressure electrical transport measurements and structural analysis reveal that the high-pressure phase of CsCl-type cubic structure exhibits the robustness of Tc against external pressure. Molecular dynamics simulation and density functional theory calculation suggest that the glassy atomic vibration characteristic mainly contributes to the appearance of the robustness. This insight accelerates the further development of unique properties within HEA superconductors and their applications.

Critical systematic investigation of the Cd–Ce system: phase stability and Gibbs energies of formation and equilibria via thermodynamic description

Abstract The CALPHAD (CAlculation of PHAse Diagrams) technique is used in the critical remodeling of the Cd–Ce system. On the basis of new experimental data in the literature, the excess Gibbs energies of the solution phase expression (liquid, bcc, fcc, and hcp_A3) are described using the Redlich–Kister equation. Four compounds (Cd3Ce, Cd6Ce, Cd11Ce, and Cd17Ce2) are treated as stochiometric compounds. Two intermetallic compounds (Cd2Ce and Cd58Ce13), which exhibit a little homogeneity range, are treated as a two-sublattice model. Two thermodynamic models are used for the CdCe and bcc. Model I is to model the compound CdCe and bcc-Ce separately. Model II is to use the formula (Cd, Ce)0.5(Cd, Ce)0.5(Va)3 to describe the compound CdCe with a CsCl-type structure (B2) and cope with the disorder–order transition from bcc-A2 to bcc-B2. The present work shows that four eutectic reactions, three peritectic reactions, two eutectoid reactions, one peritectoid transformation and three congruent reactions are observed, and the stoichiometric compound Cd17Ce2 is only stable from 804 to 882 °C in the Cd–Ce system.

Physical properties of different polymorphs of sulfur doped ZnO studied in the framework of first-principles approaches

Various physical properties of pure and sulfur-doped zinc oxide in the germanium phosphide (GeP), cesium chloride (CsCl), and nickel arsenide (NiAs) type structures are studied using the first-principles approaches in this research. The structural characteristics of the ZnOS alloys in GeP, CsCl, and NiAs type polymorphs show a slight deviation from Vegard's law, which is consistent with the available literature. It is observed that the electronic band structures of the ZnOS alloys show an exciting impact of sulfur (S) substitution in the ZnO. The ZnOS alloys, in CsCl type structure, display their metallic character. The static dielectric constants of the GeP, CsCl, and NiAs type’s polymorphs demonstrate that with increasing S concentration, polarization is enhanced. Moreover, the CsCl type phase of the ZnOS alloys exhibits the maximum value of the static dielectric constant along with showing metallic character. The optical results highlight the potential of S-doped ZnO in these structures as the highest absorption, reflectivity, and conductivity peaks shift towards low photon energies. The examination of the absorption spectra demonstrates that GeP-type ZnOS alloys are suitable for infrared to visible regime applications. On the other hand, NiAs-type ZnOS alloys are appropriate for use in the visible light regime.

Al0.88Cu0.94Fe0.18.

The inter-metallic phase with composition Al0.88Cu0.94Fe0.18 was synthesized by high-temperature sinter-ing of a mixture with initial chemical composition Al78Cu48Fe13. Al0.88Cu0.94Fe0.18 adopts the CsCl structure type in space-group Pm m. The structure analysis revealed that one site is co-occupied by Al and Cu with a ratio of 0.88 (5):0.12 (5) and the other is co-occupied by Fe and Cu with a ratio of 0.2 (4):0.8 (4). The Al/Cu⋯Fe/Cu separation is 2.5465 (13) Å.

Theoretical Investigation of Pressure Dependent Structural and Elastic Properties of MnSi Compound

MnSi is a metallic compound that exhibits a structural phase transition as a function of pressure. At ambient conditions, MnSi has a cubic structure such as NaCl type crystal structure. However, at high pressures, it undergoes a structural phase transition to a CsCl type crystal structure. The pressure-dependent structural phase transition in MnSi has been studied using various experimental and theoretical techniques. In the present work, we have used an efficient inter-ionic potential approach to predict pressure dependent structural phase change and associated volume collapse in MnSi. Therein, the potential includes the long-range Coulomb, van der Waals (vdW) interaction, and the short-range repulsive interaction up to second neighbour ions. It has been demonstrated that the Hafemeister and Flygare approach successfully evaluates the equation of state for change in volume with respect to applied pressure. We have identified a structural phase transition from B1(NaCl) type structure to B2 (CsCl) type structure in this compound. The estimated value of phase transition pressure (Pt) is 42 GPa, which is consistent with the available reported data. The identified first order phase transformation showed a volume collapse of about 10% in the vicinity of transition. In addition, we have also investigated second order of elastic constants for MnSi compound.

Pressure-induced phase transitions and metallization in layered SnSe

The group IV–VI monochalcogenides have attracted widespread attention because of their diverse physical properties and promising applications in electronics and optoelectronics. As a typical IV–VI semiconductor, SnSe displays ultra-low thermal conductivity and excellent thermoelectric properties, which deeply depends on its layered structure. The layered crystal structure and associated physical properties are sensitive to external pressure. Here, we have systematically investigated the structural behaviors and optical and electrical properties of layered SnSe under high pressure. The SnSe transforms from Pnma phase to Cmcm phase above 10 GPa, and a CsCl-type structure with a space group of Pm3¯m emerges around 30 GPa and coexists with Cmcm phase up to 42.5 GPa. The optical bandgap of SnSe shows gradual narrowing with increasing pressure, indicating gradual metallization of SnSe under compression. The pressure-induced metallization of SnSe is verified by electric transport experiments. The initial semiconducting SnSe transforms to a metallic state with increasing pressure up to 9.8 GPa. Both phase transitions and optical and electrical properties of SnSe at high pressure are reversible after releasing pressure. Our study provides a modulation strategy of crystal structures and physical properties for the group IV–VI monochalcogenides to broaden their applications in thermoelectric and optoelectronic fields.

Comment on: “The structural, elastic, electronic and optical properties of MgCu under pressure: A first-principles study”

Rahman et al. [International Journal of Mod. Phys. B 30, 1650199 (2016)] found that MgCu intermetallic compound with CsCl-type (B2) structure becomes unstable under pressure at about 15[Formula: see text]GPa. This prediction has been made using an elastic instability condition for second-order elastic constants given by Born. They determined the elastic constants using Hooke’s law for the linear stress–strain relationship which does not hold good at high pressures. It has been shown here that the values of elastic constants for MgCu at high pressures determined by Rahman et al. are not consistent with the values of bulk modulus computed from the Birch–Murnaghan equation of state (BM-EOS). We have used the Tallon formulation based on the thermodynamics of elastic deformation to estimate elastic constants which yields results for the bulk modulus of MgCu at different pressures in good agreement with those determined from the BM-EOS. It has been found that MgCu remains stable up to about 20 GPa which is significantly higher than the limit of elastic stability predicted by Rahman et al.

Structure-driven electron transition in cerium mononitride: Insights from density functional theory combined with dynamical mean-field theory

A first principle calculation is implemented with the NaCl-type and CsCl-type structures, respectively, to reveal the structure-driven 4f electron transition in cerium monoarsenide (CeN). It is found that, for the NaCl-type structure, with increasing the lattice constants, the j=5/2 manifold undergoes the insulating→metallic transition, while the j=7/2 state maintains the insulating regime. Quasi-particle weights indicate that the electronic properties of the j=5/2 manifold are significantly renormalized by the crystal structure and/or lattice constant. 4f1 atomic configuration dominates the average occupation numbers of 4f electrons for both NaCl-type and CsCl-type structures. The structure-occupation diagram suggests that the electronic properties of 4f electrons could be modulated in terms of the crystal structure (or phase transition) and/or lattice constant (or pressure).

A comment on the paper “The structural, elastic, electronic and optical properties of MgCu under pressure: A first-principles study”

Rahman et al. [International Journal of Mod. Phys. B 30, 1650199 (2016)] found that MgCu intermetallic compound with CsCl-type (B2) structure becomes unstable under pressure at about 15[Formula: see text]GPa. This prediction has been made using an elastic instability condition for second-order elastic constants given by Born. They determined the elastic constants using Hooke’s law for the linear stress–strain relationship which does not hold good at high pressures. It has been shown here that the values of elastic constants for MgCu at high pressures determined by Rahman et al. are not consistent with the values of bulk modulus computed from the Birch–Murnaghan equation of state (BM-EOS). We have used the Tallon formulation based on the thermodynamics of elastic deformation to estimate elastic constants which yields results for the bulk modulus of MgCu at different pressures in good agreement with those determined from the BM-EOS. It has been found that MgCu remains stable up to about 20 GPa which is significantly higher than the limit of elastic stability predicted by Rahman et al.

Phase formation behavior and hydrogen sorption characteristics of TiFe0.8Mn0.2 powders prepared by gas atomization

In this article we have investigated the phase formation behavior and hydrogen storage characteristics of TiFe0.8Mn0.2 samples produced by electrode induction melting gas atomization (EIGA) as well as conventional vacuum arc-melting (VAM). TiFe (46Ti–46Fe–8Mn (wt%) with cubic, CsCl-type structure) and C14 Laves (37Ti–51Fe–12Mn (wt%) with hexagonal, MgZn2-type structure) phases were detected in both atomized powders and crushed ingots, the latter possessing a lower fraction of Laves phase. Microstructural characterization included electron probe micro-analysis (EPMA) and transmission electron microscopy (TEM), which evidenced the existence of a non-equilibrium Ti2Fe phase in the atomized powders. The formation of this additional secondary phase was attributed to the rapid cooling rate during powder manufacturing, based on earlier findings and phase formation thermodynamic calculations combined with Scheil solidification simulations. In addition, kinetics and pressure-composition isotherms (PCIs) were acquired to determine both absorption and desorption enthalpy/entropy, as well as to study the influence of the synthesis route (and hence the resulting microstructure) on kinetics, thermodynamics and reaction mechanisms of the TiFe0.8Mn0.2 alloy.

Glassy atomic vibrations and blurry electronic structures created by local structural disorders in high-entropy metal telluride superconductors

Recently, robustness of superconductivity to external pressure has been observed in high-entropy (HE) metal telluride (MTe). Here, we investigated the atomic displacement parameters (Uiso), atomic vibration characteristics, and electronic structure of MTe with various configurational entropies of mixing (ΔSmix) at the M site to understand the origin of the robustness of the superconductivity. Uiso clearly increased by M-site alloying with ΔSmix ≥ 1.1R, which is evidence of the created local disorder in the NaCl-type (low-pressure) phases. The simulated vibrational density of states (DOS) shows remarkable broadening with ΔSmix ≥ 1.1R, which indicates the glassy characteristics of atomic vibrations in the NaCl-type phases. In the calculated electronic structure for the CsCl-type (high-pressure) phases, a blurry electronic band structure appears with increasing ΔSmix, which indicates the evolution of blurry electronic states in HE MTe with the CsCl-type structure. The estimated electronic DOS at the Fermi energy cannot explain the changes in Tc for HE MTe when assuming conventional electron-phonon superconductivity, but the conventional explanation seems to work for PbTe. Therefore, the pairing mechanisms in HE MTe are affected by the glassy phonon and/or blurry electronic states, and the robustness of superconductivity possibly originates from the unique electron-phonon coupling.

Molten-salt synthesis of manganese-doped intermetallic TiFexMn(1−x) nanoparticles from oxide precursors

Manganese-doped intermetallic TiFexMn(1−x) nanoparticles (NPs) were synthesized by a molten-salt synthesis method using TiO2, FeSO4•7H2O, and Mn(NO3)2•6H2O as the raw materials. The Ti–Fe–Mn oxide precursors prepared from the raw materials were efficiently reduced at temperatures as low as 600 °C in molten LiCl in the presence of CaH2 as the reducing agent. This resulted in the formation of TiFexMn(1−x) NPs exhibiting few impurities (e.g., TiFe2 or/and TiFe39). Increase in the Mn content led to peak shifts in the X-ray diffraction (XRD) patterns, indicating good incorporation of Mn into the cubic CsCl-type structure of intermetallic TiFe to form a solid solution. Nano-sized particles of< 100 nm were clearly observed in the obtained powders by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Moreover, homogeneous distribution of the constituent elements (i.e., Ti, Fe, and Mn) was confirmed by energy dispersive X-ray (EDX) spectroscopy. Finally, the hydrogen absorption properties of the prepared TiFe0.7Mn0.3 NPs were analyzed. Notably, with the exception of micron-sized TiFe0.7Mn0.3 prepared by common arc melting, the generated NPs exhibited almost no hydrogen absorption. The results obtained herein demonstrated the importance of particle size in activating TiFe-based hydrogen absorption materials.

Pressure-induced superconductivity in the weak topological insulator BiSe

Layered BiSe in the trigonal $P\overline{3}m1$ phase is a weak topological insulator and a candidate topological crystalline insulator. Here using structural, spectroscopic, resistance measurements at high pressure and density functional theory calculations, we report that BiSe exhibits a rich phase diagram with the emergence of superconductivity above 7 GPa. Structural transitions into SnSe-type energetically tangled orthorhombic structures and, subsequently, into a CsCl-type cubic structure having distinct superconducting properties are identified at 8 and 13 GPa, respectively. Superconductivity is preserved as the system transforms back to the trigonal phase upon release of pressure. Spin-orbit coupling plays a significant role in enhancement of ${T}_{c}$ in the trigonal and cubic phases. In the orthorhombic $Cmcm$ phase, ${T}_{c}$ decreases monotonically with increasing pressure, whereas unusual pressure-independent ${T}_{c}$ is observed in the cubic $Pm\overline{3}m$ phase. Theoretical analysis reveals topological surface states in the cubic phase. The emergence of superconductivity within the topological phases makes BiSe a candidate topological superconductor.