Cmcm Structure Research Articles

Pressure-induced metallic phase transition in gallium arsenide up to 24.3 GPa under hydrostatic conditions

A series of experiments of structural, vibrational and electrical transport characterization of gallium arsenide (GaAs) have been performed up to 24.3 GPa under hydrostatic conditions in a diamond anvil cell (DAC) in conjunction with in situ Raman scattering spectroscopy, electrical conductivity measurements, high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Upon compression, a phase transition from the zinc-blende (zb) to the orthorhombic (Cmcm) structure of GaAs was observed at 12.2 GPa through the discontinuous variations of the Raman shifts, Raman full-width at half-maximum values and electrical conductivity. Furthermore, the results of variable temperature electrical conductivity experiments confirmed that the high-pressure phase (Cmcm) exhibited one obvious metallic behavior. Upon decompression, the Raman scattering results of the recovered sample under ambient conditions indicated that the phase transition was reversible under hydrostatic conditions. The reversibility of the phase transition was further verified by HRTEM and AFM images for the recovered sample.

Theoretical investigations on HgTe chalcogenide materials under high pressure

First principle computations based on Density Functional Theory are made to investigate structural, mechanical, electronic properties and phase transition behaviors of HgTe chalcogenide material. It is obtained that the phase transition sequence of HgTe material is Zincblende → Cinnabar → Rocksalt → Cmcm → CsCl at 2.35 GPa, 5.95 GPa, 14.29 GPa, 52.37 GPa, respectively. The negative and values of the CsCl structure under ambient conditions indicate that this structure is mechanically unstable against the applied deformations. The small values of the elastic constants of HgTe in the cinnabar structure reveal that this structure is less resistant to deformations. The low value of tetragonal shear constant Cs of the parent phase reflects the elastic instability leading to a pressure-induced phase transition. According to our isotropic mechanical results obtained from the Voigt-Reuss-Hill (VRH) approach, the ductility behavior improves in the rocksalt and Cmcm structures which are high-pressure phases. The zincblende and cinnabar structures show semimetal and semiconductor properties in the electronic band calculations, respectively, while rocksalt, Cmcm and CsCl structures are found to be of metallic character.

Screening for O phase in advanced γ–TiAl alloys

This is a screening study using high-energy X-ray diffraction measurements to determine whether an orthorhombic phase forms in γ-based TiAl alloys of different compositions. The 13 alloy compositions investigated were chosen to be either close to commercial alloys or to identify the effects of different single alloying elements on the formation of orthorhombic phase. The orthorhombic O phase with Cmcm structure was found in several of those γ-TiAl alloys after an aging heat treatment at 550 °C for 20 h. The presence of different β-stabilising elements such as niobium, tantalum, molybdenum or vanadium did promote the formation of orthorhombic phase, while micro alloying elements such as carbon or boron were neutral in this respect. Furthermore, a limit for aluminium was also found, below which the orthorhombic O phase is formed in the alloys investigated. This limit lies between 46 at.% and 47 at.%.

The Hexagonal ↔ Orthorhombic Structural Phase Transition in Claringbullite, Cu4FCl(OH)6

ABSTRACT Frustrated magnetic phases have been a perennial interest to theoreticians wishing to understand the energetics and behavior of quasi-chaotic systems at the quantum level. This behavior also has potentially wide applications to developing quantum data-storage devices. Several minerals are examples of such phases. Since the definition of herbertsmithite, Cu3ZnCl2(OH)6, as a new mineral in 2004 and the rapid realization of the significance of its structure as a frustrated antiferromagnetic phase with a triangular magnetic lattice, there has been intense study of its magnetic properties and those of synthetic compositional variants. In the past five years it has been recognized that the layered copper hydroxyhalides barlowite, Cu4BrF(OH)6, and claringbullite, Cu4FCl(OH)6, are also the parent structures of a family of kagome phases, as they also have triangular magnetic lattices. This paper concerns the structural behavior of claringbullite that is a precursor to the novel frustrated antiferromagnetic states that occur below 30 K in these minerals. The reversible hexagonal (P63/mmc) ↔ orthorhombic (Pnma or Cmcm) structural phase transition in barlowite at 200−270 K has been known for several years, but the details of the structural changes that occur through the transition have been largely unexplored, with the focus instead being on quantifying the low-temperature magnetic behavior of the orthorhombic phase. This paper reports the details of the structural phase transition in natural claringbullite at 100−293 K as studied by single-crystal X-ray diffraction. The transition temperature has been determined to lie between 270 and 293 K. The progressive disordering of Cu at the unusual trigonal prismatic Cu(OH)6 site on heating is quantified through the phase transition for the first time, and a methodology for refining this disorder is presented. Key changes in the behavior of Cu(OH)4Cl2 octahedra in claringbullite have been identified that suggest why the Pnma structure is likely stabilized over an alternative Cmcm structure. It is proposed that the presence of a non-centrosymmetric octahedron in the Pnma structure allows more effective structural relaxation during the phase transition than can be achieved by the Cmcm structure, which has only centrosymmetric octahedra.

Pressure-induced phase transitions in the topological crystalline insulator SnTe and its comparison with semiconducting SnSe: Raman and first-principles studies

SnTe is a narrow band-gap topological crystalline insulator (TCI), whereas SnSe is a normal semiconductor. We report Raman study of SnTe and SnSe as a function of pressure at room temperature along with first-principles density functional theory calculations. Under pressure, isostructural transition is observed in SnTe, as revealed by the anomalous softening of the strongest Raman mode up to 1.5 GPa, accompanied by an increase in the linewidth. Our first-principles calculations show that the mirror Chern number of SnTe does not change and the TCI phase remains unaffected by pressure. Raman signatures of its phase transition at 1.5 GPa are associated with phonon instability at the $\mathrm{\ensuremath{\Gamma}}$ point and inversion of the lowest-energy conduction bands. An anomaly in the electron-phonon coupling results in anomalous behavior of the Raman modes at this pressure. Further, SnTe undergoes structural transitions at $\ensuremath{\sim}5.8, \ensuremath{\sim}12$, and $\ensuremath{\sim}18.3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The 5.8-GPa transition is associated with a structural transition from the ambient cubic ( $Fm\overline{3}m$ ) to orthorhombic (Pnma) phase, which is no longer a topological insulator, resulting in a topological phase transition. Above the transition pressure of 12 GPa, another orthorhombic Pnma[GeS] phase is stabilized, coexisting with the Pnma phase. The reduction in the number of observed Raman modes above $\ensuremath{\sim}18.3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and enthalpy calculations show a transition from orthorhombic (Pnma) to a more symmetric cubic ($\mathrm{Pm}\overline{3}\mathrm{m}$) structure. Our high-pressure study of SnSe, on the other hand, reveals that it undergoes two phase transitions: one from the orthorhombic (Pnma) structure to the orthorhombic (Cmcm) structure at $\ensuremath{\sim}6.2\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and the other at $\ensuremath{\sim}12.9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, in which the Cmcm phase undergoes a semimetal to metal transition. Density functional theory calculations capture the contrast in the pressure-dependent behavior of the topological crystalline insulator SnTe and the normal semiconductor SnSe.

Structure prediction, high pressure effect and properties investigation of superhard B6O

Ab initio data mining approach has been used in order to investigate B6O system and discover new possible modifications, besides experimentally known R-3m (α-B6O) structure and theoretically predicted Cmcm (β-B6O) structure. DFT calculations were performed by two different functionals, LDA and PBE. In this work, we focus on the structure, mechanical, and electronic properties of the experimentally known α-B6O structure and newly predicted modifications with the B6O stoichiometry. Moreover, mechanical properties including elastic constants, bulk, shear and elastic moduli, Poisson’s ratio, Pugh’s criterion, and hardness are given for the investigated modifications of B6O. In particular, we have investigated the influence of the high pressure on the electronic and mechanical properties. Results of our study provide more insight in the B6O superhard material and open new possibilities for various device applications.

Synthesis, structural and spectroscopic properties of orthorhombic compounds BaLnCuS3 (Ln = Pr, Sm)

Abstract Ternary sulfides BaPrCuS3 and BaSmCuS3 are first synthesized by the sulphidation reaction of a mixture of related oxides and metal Cu in a flow of (CS2, H2S) at 1170 K. The crystal structures of BaPrCuS3 and BaSmCuS3 are obtained by Rietveld method. BaPrCuS3 crystallizes in space group Pnma with unit cell parameters a = 10.56074(6), b = 4.11305(2) and c = 13.42845(7) A, V = 583.289 (5) A3, Z = 2 (structure type Eu2CuS3). BaSmCuS3 crystallizes in space group Cmcm with unit cell parameters a = 4.07269(4), b = 13.4499(1) and c = 10.3704(1) A, V = 568.06 (1) A3, Z = 2 (structure type KZrCuS3). The structural model is proposed for the Cmcm→Pnma transition in ABCX3 (X = S, Se) compounds for the sequence Sm-Pm-Nd-Pr. The dimensionless tolerance factor t = IR(A) × IR(C)/IR(B)2 is suggested to control the boundary between the Cmcm and Pnma structures. The micromorphological, thermal and spectroscopic properties are evaluated for BaPrCuS3. The compound melts incongruently at Tmelt = 1580.9 K. In BaPrCuS3, the band gap is estimated to be 2.1 eV. The vibrational parameters of BaPrCuS3 and BaSmCuS3 are comparatively observed by Raman spectroscopy.

Energetics of Formation and Disordering in Rare Earth Weberite RE3TaO7 Materials.

The recent finding of local weberite-like ordered domains in disordered and radiation damaged pyrochlore oxides has sparked interest in studying the structure, stability, and order-disorder in compounds that form in the weberite structure. In order to understand the relationships among the energetics, structure, and disordering, weberites of the formula RE3TaO7 (RE = La, Nd, Sm-Yb) were synthesized by conventional solid-state techniques. High temperature oxide melt solution calorimetry was used to determine their enthalpies of formation. Rietveld refinement of PXRD patterns shows that the La compound forms in the weberite La3NbO7 (Cmcm) structure; the Nd compound has both Y3TaO7 (C2221)-type and La3NbO7-type polymorphs; the Sm-Ho compounds crystallize in the weberite Y3TaO7 (C2221) structure; and the Ho-Yb compounds adopt the defect fluorite (Fm3̅m) disordered structure. Depending on the reaction temperature, Ho3TaO7 crystallizes in ordered Y3TaO7 (low temperature) or disordered defect fluorite (high temperature) structures. The formation enthalpy of weberites becomes more exothermic with increasing rare earth ionic radius, implying an increase in stability, i.e., La3TaO7 is most stable and Yb3TaO7 is least stable with respect to the component oxides. The calorimetric data also show that ordered Ho3TaO7 (Y3TaO7 structure) is energetically more stable by 9.2 ± 1.1 kJ/mol than disordered Ho3TaO7 (defect fluorite structure).

Experimental and computational modelling study of Ni substitution for Fe in Zr3Fe and its hydride

Zr3Fe and Ni-substituted Zr3Fe alloys with 30 and 50 at.% Ni were synthesized and their hydrogen absorption/desorption characteristics were compared experimentally (pressure–composition isotherms, thermal desorption spectroscopy, in-situ neutron diffraction) and by computational methods (ab-initio molecular dynamics (AMD), nudged elastic band theory (NEB)). All the alloys absorbed hydrogen to a hydrogen-to-metal atomic ratio of about 1.7, but the hydrides formed were stable at room temperature. The Zr3Fe0.5Ni0.5 alloy and its hydrided form were multi-phase. The Zr3Fe0.7Ni0.3 alloy was single-phase and retained the Cmcm structure of the parent intermetallic. In-situ neutron diffraction with D2 in place of H2 showed that the hydride formed in the isotherm measurements, Zr3Fe0.7Ni0.3H6.88, had the same structure (Cmcm) as Zr3FeH7, while disproportionation was observed in the hydrogenation of Zr3Fe. The kinetics of hydride formation was slower in both the Ni-substituted alloys. Thermal desorption spectroscopy showed that substitution of 0.3Ni significantly destabilized the hydride, lowering the temperature of the principal desorption peak by about 300 K relative to Zr3Fe–H2, without loss of hydrogen capacity, and avoiding disproportionation. Based on the structures determined by neutron diffraction, AMD and NEB calculations were conducted to compare Zr3Fe and Zr3Fe0.7Ni0.3 and their hydrides. The AMD calculations predicted that H diffusion was slower in Ni-substituted Zr3Fe, in agreement with the experimental observation of slower kinetics, implying a higher activation energy for H migration. The NEB calculations also predicted a higher energy barrier for H migration in Ni-substituted Zr3Fe.

Instability of the layered orthorhombic post-perovskite phase of SrTiO3 and other candidate orthorhombic phases under pressure

While the tetragonal antiferro-electrically distorted (AFD) phase with space group I4/mcm is well known for SrTiO3 to occur below 105 K, there are also some hints in the literature of an orthorhombic phase, either at the lower temperature or at high pressure. A previously proposed orthorhombic layered structure of SrTiO3, known as the post-perovskite or CaIrO3 structure with space group Cmcm is shown to have significantly higher energy than the cubic or tetragonal phase and to have its minimum volume at larger volume than cubic perovskite. The Cmcm structure is thus ruled out. We also study an alternative Pnma phase obtained by two octahedral rotations about different axes. This phase is found to have slightly lower energy than the I4/mcm phase in spite of the fact that its parent, in-phase tilted P4/mbm phase is not found to occur. Our calculated enthalpies of formation show that the I4/mcm phase occurs at slightly higher volume than the cubic phase and has a negative transition pressure relative to the cubic phase, which suggests that it does not correspond to the high-pressure tetragonal phase. The enthalpy of the Pnma phase is almost indistinguishable from the I4/mcm phase. Alternative ferro-electric tetragonal and orthorhombic structures previously suggested in literature are discussed.

Phosphorus Dimerization in Gallium Phosphide at High Pressure

Using combined experimental and computational approaches, we show that at 43 GPa and 1300 K gallium phosphide adopts the super- Cmcm structure, here indicated with its Pearson notation oS24. First-principles enthalpy calculations demonstrate that this structure is more thermodynamically stable above ∼20 GPa than previously proposed polymorphs. In contrast to other polymorphs, the oS24 phase shows a strong bonding differentiation and distorted fivefold coordination geometries of both P atoms. The shortest bond of the phase is a single covalent P-P bond measuring 2.171(11) Å at synthesis pressure. Phosphorus dimerization in GaP sheds light on the nature of the super- Cmcm phase and provides critical new insights into the high-pressure polymorphism of octet semiconductors. Bond directionality and anisotropy explain the relatively low symmetry of this high-pressure phase.

High-pressure phases of Weyl semimetals NbP, NbAs, TaP, and TaAs

The high pressure phase diagrams of TaAs family (NbP, NbAs, TaP and TaAs) are explored systematically using crystal structure search together with first-principles calculations. Our calculations show that NbAs and TaAs have similar phase diagrams and possess the same structural phase transition sequence of I41md $\rightarrow$ P-6m2 $\rightarrow$ P21/c $\rightarrow$ Pm-3m, with a bit different transition pressures. The phase transition sequence of NbP and TaP are somewhat different from that of NbAs and TaAs. In which new structures emerge, for instance, the Cmcm structure in NbP and the Pmmn structure in TaP. It is found interestingly that in the electronic structure of the high-pressure phase P-6m2-NbAs, there are Weyl points and triple degenerate points coexisted, similar to the high-pressure P-6m2-TaAs.

Phase transformation of GaAs at high pressures and temperatures

The high-pressure behavior of gallium arsenide, GaAs, has been investigated using an in-situ X-ray powder diffraction technique in a diamond anvil cell combined with a resistance heating method, at pressures and temperatures up to 25 GPa and 1000 K respectively. The pressure-induced phase transition from a zincblende to an orthorhombic (Cmcm) structure was observed. This transition occurred at 17.3 GPa and at room temperature, where a negative temperature dependence for this transition was confirmed. The transition boundary was determined to be P (GPa) = 18.0 − 0.0025 × T (K).

Effect of structural phase transformations under pressure on electronic and optical properties of CuInS2

First-principles plane-wave calculations were employed to study the phase transitions, electronic and optical properties of chalcopyrite CuInS2 (CIS) under pressure, which is a promising semiconductor compound material for nonlinear optical, photovoltaic and bio-applications. From the variations of Gibbs free energy and volume with pressure, we confirmed an experimentally found phase transformation of CIS from tetragonal I4¯2d structure into the cubic Fm3¯m phase. It occurs at 7.9 GPa with a volume reduction of 15.6%, which are comparable with the experimental values of 9.5 GPa and 12%. A new possible phase transition of CIS from the cubic Fm3¯m structure to the orthorhombic Cmcm structure under higher pressure was predicted occurring at 49.6 GPa with a volume reduction about 0.64%. Using the more accurate HSE06 functional, we found that the growth rate of energy gap with pressure (dEg/dP) for the I4¯2d phase is about 24.8 meV/GPa, very close to the experimental value of 24 meV/GPa. The calculated electronic properties for Fm3¯m and Cmcm phases show that CIS is a metallic material under high pressure, rather than a semiconductor. The optical absorption spectra indicate that with the increase of pressure the absorption edge of CIS becomes smaller while the optical energy gap is larger.

First-principles investigation on crystal structure and physical properties of HfB4

Applying the evolutionary algorithm for crystal structure prediction and first-principles calculations, a newly orthorhombic Cmcm structure of HfB4 is predicted theoretically. The result shows that this new phase is much more superior to the early proposed YB4-, ReP4-, FeB4-, CrB4-, and MnB4-type structures. The calculations of the phonon dispersion curve and elastic constants verify that the predicted Cmcm phase is dynamically and mechanically stable. Moreover, the predicted large shear modulus (240 GPa) and high hardness (45.7 GPa) imply that the Cmcm-HfB4 phase is a potential superhard material. At the same time, the dependences of the Young's modulus, bulk modulus, and shear modulus for HfB4 on the crystal orientation are also systematically studied. Further analyses of the electronic structure and chemical bonding properties indicate that the strong BB and BHf covalent bonds make great contributions to its high hardness and stability.

The crystal structure of lueshite at 298 K resolved by high-resolution time-of-flight neutron powder diffraction

Refinement of time-of-flight high-resolution neutron powder diffraction data for lueshite (Na, Ca)(Nb, Ta, Ti)O3, the natural analogue of synthetic NaNbO3, demonstrates that lueshite at room temperature (298 K) adopts an orthorhombic structure with a 2a p × 2a p × 4a p superlattice described by space group Pmmn [#59: a = 7.8032(4) A; b = 7.8193(4) A; c = 15.6156(9) A]. This structure is analogous to that of phase S of synthetic NaNbO3 observed at 753–783 K (480–510 °C). In common with synthetic NaNbO3, lueshite exhibits a series of phase transitions with decreasing temperature from a cubic ( $$Pm\bar{3}m$$ ) aristotype through tetragonal (P4/mbm) and orthorhombic (Cmcm) structures. However, the further sequence of phase transitions differs in that for lueshite the series terminates with the room temperature S (Pmmn) phase, and the R (Pmmn or Pnma) and P (Pbcm) phases of NaNbO3 are not observed. The appearance of the S phase in lueshite at a lower temperature, relative to that of NaNbO3, is attributable to the effects of solid solution of Ti, Ta and Ca in lueshite.

High-Pressure Study of Perovskites and Postperovskites in the (Mg,Fe)GeO3 System.

The effect of incorporation of Fe2+ on the perovskite (Pbnm) and postperovskite (Cmcm) structures was investigated in the (Mg,Fe)GeO3 system at high pressures and temperatures using laser-heated diamond anvil cell and synchrotron X-ray diffraction. Samples with compositions of Mg# ≥ 48 were shown to transform to the perovskite (∼30 GPa and ∼1500 K) and postperovskite (>55 GPa, ∼1600-1800 K) structures. Compositions with Mg# ≥ 78 formed single-phase perovskite and postperovskite, whereas those with Mg# < 78 showed evidence for partial decomposition. The incorporation of Fe into the perovskite structure causes a decrease in octahedral distortion as well as a modest decrease in bulk modulus (K0) and a modest increase in zero-pressure volume (V0). It also leads to a decrease in the perovskite-to-postperovskite phase transition pressure by ∼9.5 GPa over compositions from Mg#78 to Mg#100.

Dense Carbon Monoxide to 160 GPa: Stepwise Polymerization to Two-Dimensional Layered Solid

Carbon monoxide (CO) is the first molecular system found to transform into a nonmolecular “polymeric” solid above 5.5 GPa, yet been studied beyond 10 GPa. Here, we show a series of pressure-induced phase transformations in CO to 160 GPa: from a molecular solid to a highly colored, low-density polymeric phase I to translucent, high-density phase II to transparent, layered phase III. The properties of these phases are consistent with those expected from recently predicted 1D P21/m, 3D I212121, and 2D Cmcm structures, respectively. Thus, the present results advocate a stepwise polymerization of CO triple bonds to ultimately a 2D singly bonded layer structure with an enhanced ionic character.

Structural phase transition in polycrystalline SnSe: a neutron diffraction study in correlation with thermoelectric properties

SnSe has been recently reported as a promising and highly efficient thermoelectric intermetallic alloy. The present material has been prepared by arc melting, as mechanically robust pellets, consisting of highly oriented polycrystals. The evolution of its orthorhombic GeS-type structure (space group Pnma) and phase transition to TlI-type structure (space group Cmcm) at high temperature has been studied in situ by neutron powder diffraction (NPD) in the temperature range 295–873 K. This transition has been identified by differential scanning calorimetry measurements, yielding sharp peaks at 795 K. In addition, thermal transport properties were measured in a similar temperature range, and large Seebeck coefficients, as high as 1050 µV K−1 at 625 K, were found. The analysis from NPD data demonstrates an almost perfect stoichiometry, Sn0.998(8)Se, that does not evolve with temperature, and a progressive decrease of the anharmonicity of the chemical bonds upon entering the domain of the Cmcm structure.

Structural Phase Transitions of ZnTe under High Pressure Using Experiments and Calculations**Supported by the National Natural Science Foundation of China under Grant No 11474280, the National Basic Research Program of China under Grant No 2011CB808200, and the Chinese Academy of Sciences under Grant Nos KJCX2-SW-N20 and KJCX2-SW-N03.

The pressure-induced structural transitions of ZnTe are investigated at pressures up to 59.2 GPa in a diamond anvil cell by using synchrotron powder x-ray diffraction method. A phase transition from the initial zinc blende (ZB, ZnTe-I) structure to a cinnabar phase (ZnTe-II) is observed at 9.6 GPa, followed by a high pressure orthorhombic phase (ZnTe-III) with Cmcm symmetry at 12.1 GPa. The ZB, cinnabar (space group P3121), Cmcm, P31 and rock salt structures of ZnTe are investigated by using density functional theory calculations. Based on the experiments and calculations, the ZnTe-II phase is determined to have a cinnabar structure rather than a P31 symmetry.