Cmcm Structure Research Articles

First-principles structural design of superhard material of ZrB4

Using the first-principles calypso algorithm for crystal structure prediction, we have predicted two orthorhombic Cmcm and Amm2 structures of ZrB4, which are energetically much superior to the previously proposed WB4-, CrB4-, and MoB4-type structures and stable against decompression into a mixture of Zr and B at ambient pressure. The two orthorhombic structures consist of a hexagonal B ring and ZrB12 units connected by edges and one hexagonal B ring in Cmcm and Amm2 structure, respectively. The calculated large shear modulus (e.g., 229 GPa) and high hardness (42.8 GPa for Cmcm and 42.6 GPa for Amm2) reveal that they are potentially superhard materials. The high hardness is attributed to a stacking of B-Zr-B "sandwiches" layers linked by strong covalent B-B bonding.

First-principles investigation of structural phase transitions and electronic properties of CuGaSe2 up to 100 GPa

First-principles calculations based on density functional theory have been carried out for the photovoltaic material CuGaSe2 up to 100GPa to clarify its possible structural phase transitions and electronic properties. Energy–Volume analysis confirms that CuGaSe2 transforms from the tetragonal I4¯2d structure to the cubic Fm3¯m structure at 11.87GPa with a volume reduction of 13.33%, which are in fair agreement with the experimental results of 13GPa and 13%, respectively. We also predict another phase transformation of CuGaSe2 from the cubic Fm3¯m structure to the orthorhombic Cmcm structure at 51.4GPa with a volume reduction of 0.49%. By using the HSE functional for Gap–Pressure analysis, we find that the energy gap of I4¯2d structure broadens at a rate of 52.3meV/GPa. By the band structure and the density of states calculation, we predict the metallic nature of the Fm3¯m and Cmcm phases and discuss the original reason of metallic nature under high pressure by the calculation of bond lengths and the charge redistribution.

Phase stability and superconductivity of strontium under pressure

We have used the ab initio random structure searching method together with density functional theory calculations to find stable structures of strontium under pressures up to 50 GPa. We predict a sequence of structural phase transitions and the stability of an orthorhombic structure of Cmcm symmetry above 25 GPa. Our energy, lattice dynamics, and molecular dynamics calculations confirm the stability of the Cmcm structure. The electron-phonon coupling calculations show that superconductivity arises in the bcc structure of compressed Sr and that it continues to exist in the Cmcm structure. The calculated superconducting transition temperatures are in good agreement with experiment. Our study gives an excellent account of the experimental observations.

Structural phase transitions in Si under hydrostatic and uniaxial compression

Phase transitions induced by hydrostatic and uniaxial compression of Si are examined using a density functional--based metadynamics method. A metastable Cmcm structure is predicted to form under hydrostatic compression at 300 K in the pressure range near 20 GPa. Enthalpy calculations indicate that this new phase of Si is energetically competitive with the other Si structures found in this pressure range. This phase is shown to be dynamically stable, with a structure closely related to that of the \ensuremath{\beta}-Sn and Imma structures previously identified in experiments. Uniaxial compression at 15 GPa and 300 K obtained simple hexagonal Imma and \ensuremath{\beta}-Sn structures. The low energy barrier for the formation of the Cmcm structure from the diamond ambient pressure form suggests that this structure may be seen in experiment with the proper experimental conditions.

Fe–C and Fe–H systems at pressures of the Earth's inner core

The solid inner core of Earth is predominantly composed of iron alloyed with several percent Ni and some lighter elements, Si, S, O, H, and C being the prime candidates. To establish the chemical composition of the inner core, it is necessary to find the range of compositions that can explain its observed characteristics. Recently, there have been a growing number of papers investigating C and H as possible light elements in the core, but the results were contradictory. Here, using ab initio simulations, we study the Fe–C and Fe–H systems at inner core pressures (330–364 GPa). Based on the evolutionary structure prediction algorithm USPEX, we have determined the lowest-enthalpy structures of all possible carbides (FeC, Fe2C, Fe3C, Fe4C, FeC2, FeC3, FeC4, Fe7C3) and hydrides (Fe4H, Fe3H, Fe2H, FeH, FeH2, FeH3, FeH4) and have found that Fe2C (space group Pnma) is the most stable iron carbide at pressures of the inner core, while FeH, FeH3, and FeH4 are the most stable iron hydrides at these conditions. For Fe3C, the cementite structure (space group Pnma) and the Cmcm structure recently found by random sampling are less stable than the I-4 and C2/m structures predicted here. We have found that FeH3 and FeH4 adopt chemically interesting thermodynamically stable crystal structures, containing trivalent iron in both compounds. We find that the density of the inner core can be matched with a reasonable concentration of carbon, 11–15 mol.% (2.6–3.7 wt.%) at relevant pressures and temperatures, yielding the upper bound to the C content in the inner core. This concentration matches that in CI carbonaceous chondrites and corresponds to the average atomic mass in the range 49.3–51.0, in close agreement with inferences from Birch's law for the inner core. Similarly made estimates for the maximum hydrogen content are unrealistically high: 17–22 mol.% (0.4–0.5 wt.%), which corresponds to the average atomic mass of the core in the range 43.8–46.5. We conclude that carbon is a better candidate light alloying element than hydrogen.

Structural, mechanical and electronic properties of ZnTe polymorphs under pressure

We have performed first principles calculations based on density functional theory (DFT) to study the structural, mechanical and electronic properties, and pressure-induced phase transition behavior of ZnTe. The generalized gradient approximation is employed together with the projector augmented wave potentials to describe the electron-ion interaction. We consider zinc blende (B3) structure as the ambient pressure phase, the cinnabar (B3), Cmcm (B33) and rocksalt (B1) structures as candidates for the high pressure phases. The calculated structural properties are in good agreement with the experiments and earlier ab initio predictions, as is the transition pressure between them. We determine the sequence of the structural phase transition of ZnTe as B3→B9→B33, which agrees well with the experiments. The pressure dependence of the elastic constants and the electronic energy band gap of both the ambient and high pressure structures are reported. Tetragonal shear elastic constant C′ takes very small value in the parent phase, indicating the elastic instability resulting in phase transition to the high pressure structure. The obtained electronic results show that zinc blende structure is the direct energy band gap semiconductor at Γ point, while the cinnabar structure has indirect energy band gap along the symmetry of Γ→K and Cmcm phase displays the metallic behavior.

Ab initio calculation of high pressure phases and electronic properties of CuInSe2

We used an ab initio method to calculate the high pressure phases of CuInSe2. By using the experimentally suggested phases, the enthalpy difference showed that the I4̄2d structure transforms into Fm3̄m at 12 GPa and then into Cmcm at 42 GPa. The volume reductions at each phase transition are 13.9% and 1.9% respectively, compared with 11% and 1% from experiments. By using the sX-LDA functional, we found that the bandgap in the I4̄2d structure increases at the rate of 39.6 meV/GPa, in fair agreement with photoabsorption experiments. The band gap is closed in the Fm3̄m and Cmcm structures. The bond lengths between Cu–Se and In–Se were investigated. We found that the bond lengths can be related to the behavior of the energy gap under high pressure. The path of transformation from Fm3̄m to Cmcm was proposed. The energy barrier between the two phases was estimated. The upper bound of the energy barrier is 17 meV which is equivalent to 198 K. This finding can explain the existence of two phases at room temperature reported by experimental study.

Pressure-induced phase transition in wurtzite ZnTe: an ab initio study

A constant pressure ab initio MD technique and density functional theory with a generalized gradient approximation (GGA) was used to study the pressure-induced phase transition in wurtzite ZnTe. A first-order phase transition from the wurtzite structure to a Cmcm structure was successfully observed in a constant-pressure molecular dynamics simulation. This phase transformation was also analyzed using enthalpy calculations. We also investigated the stability of wurtzite (WZ) and zinc-blende (ZB) phases from energy-volume calculations, and found that both structures show quite similar equations of state and transform into a Cmcm structure at 16 GPa using enthalpy calculations, in agreement with experimental observations. The transition phase, lattice parameters and bulk properties we obtained are comparable with experimental and theoretical data.

Flux growth of a new cobalt–zinc–tin ternary phase Co 7+ xZn 3− xSn 8 and its relationship to CoSn

The intermetallic compound Co 7+ x Zn 3− x Sn 8 (−0.2< x<0.2) forms from the reaction of cobalt in zinc/tin eutectic flux. This phase has a new structure type in orthorhombic space group Cmcm, with unit cell parameters a=4.138(1) Å, b=12.593(4) Å, and c=11.639(4) Å ( Z=2; R 1=0.0301). Varying the amount of cobalt in the synthesis leads to formation of a superstructure in space group Pnma, with lattice parameters a=12.5908(2) Å, b=11.6298(3) Å, and c=8.2704(2) Å ( Z=4; R 1=0.0347). A Co/Zn mixed site and a partially occupied Co site in the Cmcm structure order to form the Pnma supercell. TGA/DSC studies indicate that the binary phase CoSn initially forms in the flux at 1173 K, and then reacts with the zinc in the cooling solution to form the ternary structure at 823 K. This phase exhibits Pauli paramagnetic behavior.

The structural transition behavior of CdSe/ZnS core/shell quantum dots under high pressure

AbstractThe structural phase transition of CdSe/ZnS core/shell quantum dots (QDs) has been studied by in situ angle‐dispersive X‐ray diffraction under high pressure up to 53.6 GPa. The CdSe core transforms from wurtzite to rock‐salt structure near 6.3 GPa and then to Cmcm or distorted Cmcm structure probably occurs at 45.1 GPa which has not been observed in CdSe nanomaterials before. The critical pressure from wurtzite to rock‐salt and the bulk modulus of rock‐salt phase are much higher than those for bulky and uncapped nanoparticle CdSe. The released sample can be kept in rock‐salt phase for a certain time, verified by photoluminescence (PL) spectra, quite different from the reversible transition for pure CdSe. A reasonable interpretation of the experimental phenomena is given by comparing the bulk modulus of the core and shell and studying the stress sate of the core after decompression. Our study suggests that capping a hard shell is an effective approach to quench the high pressure phase of nanomaterial with a reversible phase transition.

The electrical properties of ZnTe under high pressure and moderate temperature

AbstractThe resistivity of ZnTe powders has been measured under high pressure up to 29 GPa and temperature ranged from 303.0 to 403.0 K by using a diamond anvil cell. The resistivity changed discontinuously at 9.5 and 13.1 GPa, corresponding to the phase transitions from zinc blende to cinnabar and then to Cmcm structure. The decrease of activation energy by one order of magnitude indicates the deep‐to‐shallow transition of acceptor levels at 6.7 GPa. The resistivity decreases with temperature increasing indicates the semiconducting character of ZnTe in zinc blende phase. The resistivity of ZnTe in cinnabar phase decreases slightly with temperature increasing, indicating that this phase is a narrow‐gap semiconductor. After 19.1 GPa, the resistivity increases with temperature increasing, indicating the metallic character of Cmcm phase. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structural phase transition of SnSe under uniaxial stress and hydrostatic pressure: an ab initio study

We study the structural behavior of SnSe under the hydrostatic pressure using a constant pressure ab initio technique. We find SnSe undergoes a structural second order phase transition from the orthorhombic (Pnma) structure to orthorhombic (Cmcm) structure in the constant pressure simulation at 7 GPa which is in good agreement with the recent experimental study. The Cmcm structure is fivefold coordinated. This phase transition is also analyzed from the total energy calculations. Besides, we study the behavior of SnSe under uniaxial stress.

Synthesis, structure and magnetic properties of V 4O 9—A missing link in binary vanadium oxides

V 4O 9: A missing link of Wadsley phases has been successfully synthesized by using sulfur as a reducing agent at a low temperature and its structure has been determined by combining electron, X-ray and neutron diffractions. V 4O 9 has an orthorhombic Cmcm structure and the lattice parameters are a=10.356(2) Å, b=8.174(1) Å and c=16.559(3) Å at room temperature. The structure is composed of shared edges and corners of three types of polyhedra; a VO 6 distorted octahedron, a VO 5 pyramid and a VO 4 tetrahedron. The structure of V 4O 9 is very similar to that of vanadyl pyrophosphate (VO) 2P 2O 7 which has PO 4 tetrahedra instead of VO 4 tetrahedra. This indicates that V 4O 9 is a salt of pyro-ion [V 2O 7] 4-; (VO) 2V 2O 7. The magnetic properties of V 4O 9 have been investigated by magnetic susceptibility, high-field magnetization and inelastic neutron scattering measurements. V 4O 9 is a quantum spin system with a spin-gapped ground state. The excitation gap between the singlet ground state and the excited triplet state is approximately 73 K. The magnetic susceptibility behavior suggests that V 4O 9 is a spin-1/2 dimer system with significant interdimer interactions, as opposed to (VO) 2P 2O 7, which is an alternating spin-1/2 chain system. This difference is thought to be due to the fact that VO 4-mediated interactions are considerably weaker than PO 4-mediated interactions.

Formation of a Cmcm phase in SnS at high pressure; an ab initio constant pressure study

The stability of SnS at high pressure is studied using a constant pressure ab initio technique. For the first time, a pressure-induced phase transformation from the Pnma structure to a Cmcm structure with the application of pressure is predicted through the simulations in this material. The Cmcm phase is still a layered structure, consisting of rocksalt-like bilayers, similar to that formed at high temperatures. The Cmcm structure is fivefold coordinated. This phase transformation gradually proceeds and is due to the significant decrease of the second neighbor distances. This phase change is also studied by total energy calculations.

First-principles study on structural properties and phase stability of III-phosphide (BP, GaP, AlP and InP)

We have performed first principles total-energy calculations based on the full-potential augmented plane-wave plus local orbitals (FP-LAPW+lo) method in order to investigate the phase transformations under high pressure of III-phosphide (BP, GaP, AlP, InP) in the zinc-blende, NaCl, CsCl, sc16, cmcm, d-β-tin, NiAs, Immm, and Imm2 structures. The generalized gradient approximation (GGA) was used for the exchange and correlation energy density functional. Our work relates directly to recent experimental work on III-phosphide compounds. The ground-state properties such as lattice parameter, bulk modulus and its pressure derivative as well as the structural phase stability. Along with previous work, we now have enough theoretical results to support a different systematics of the high-pressure phases of III-phosphide compounds.

Symmetry breaking of ionic semiconductors under pressure: the case of InAs

The NaCl-to-Cmcm phasetransition and the Cmcm structure of InAs under high pressure are studied by x-ray diffraction. The lattice parametersand fractional coordinates are given as a function of pressure. We propose a mechanismresponsible for this type of symmetry breaking under pressure. We show that theppσ interactions do not play a major role in the stabilization of the NaCl structure. Consequently theNaCl-to-Cmcm transition occurs only in compounds with a large charge transfer. General conclusions onthe behavior of III–V semiconductors under pressure are drawn.

First principles study of the relative stability and the electronic properties of GaN

Abstract A theoretical study of structural and electronic properties of the GaN compound is presented using the first-principles full-potential augmented plane wave approach within the generalized gradient approximation (GGA). Results are given for lattice parameters, bulk modulus and its first derivatives in the wurtzite, zinc-blende, rock-salt, CsCl, d-β-tin, Immm, Imm2, Cinnabar, Cmcm, and NiAs structures. The relative stability, phase transitions, band gap energies and densities of state for GaN are also presented. The results of these calculations are compared with the available theoretical and experimental data.

Crystal lattice and band structure of the intermediate high-pressure phase ofPbSe

In the present paper the results of fitting synchrotron diffraction data are obtained for theintermediate high-pressure phase (9.5 GPa) of the lead selenide based compoundPb1−xSnxSe (x = 0.125)—an optoelectronic as well as a thermoelectric material—for two types of latticesymmetries Pnma (space group #62) and Cmcm (space group #63). Both lattice parametersand positions of atoms for the above mentioned structures have been used incalculations of the electron structure of high-pressure phases. The main differencebetween the electronic properties for Cmcm and Pnma structures establishedin electronic structure calculations is that in the first one the PbSe compoundwas found to be a metal, while in the second a small semiconductor gap (EG = 0.12 eV) was obtained. Moreover, the forces in the Cmcm structure are an order of magnitudelarger than those calculated for the Pnma lattice. In the optimized, Pnma structurewithin a generalized gradient approximation (GGA), the band gap increases up toEG = 0.27 eV. The result coincides with the data on thermoelectric powerand electrical resistance data pointing to a semiconductor gap of∼0.2 eVat ∼9.5 GPa. Thus, the Pmna type of lattice seems to be a preferable version for the intermediatephase compared with the Cmcm one.

High pressure raman spectroscopic study of low-cristobalite GaPO4

Gallium orthophosphate of low-cristobalite structure has been studied by Raman spectroscopy up to 28.5 GPa. Three phase transitions have been identified: 1.6, 3.1 and ∼7 GPa. Raman spectra (200 to 700 cm-1) acquired above 3 GPa show signs of six-coordinated gallium ions, in agreement with earlier studies that the orthorhombic C2221 structure transforms into the orthorhombic Cmcm structure. As pressure increases above 8 GPa, the Raman peaks associated with the low-cristobalite framework structure diminish gradually and replaced by peaks that are possibly related to isolated phosphate units. The spectra also show that the structure of the quenched sample retains that of the high-pressure phase prior to decompression.

Elusive AuF in the solid state as accessed via high pressure comproportionation

Density Functional Theory (DFT) calculations indicate that AuF might be synthesized at 22.6 GPa from AuF3 and Au (1 : 2), and subsequently quenched down to at least 5 GPa in the Cmcm (bent chain) structure.