Cmcm Phase Research Articles

High pressure structural studies of AgInTe2

The structural phase transformations in the chalcopyrite semiconductor AgInTe2 have been studied up to 10 GPa on both pressure increase and decrease. The experiments were conducted using angle-dispersive X-ray diffraction with synchrotron radiation and an image plate. The diffraction patterns of AgInTe2 at ambient pressure reveal two coexisting phases: the first has the chalcopyrite structure while the second has a zincblende-like structure. On pressure increase both phases transformed at 3-4 GPa to a cation-disordered orthorhombic structure with spacegroup Cmcm. On pressure decrease, the chalcopyrite phase started to reappear at 0.55 GPa, and the Cmcm phase disappeared completely at ambient pressure.

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 studies of phase stability, electronic and elastic properties in BBi compound

Abstract An investigation into the structural stabilities, electronic and elastic properties of BBi was conducted by using the first-principles calculations. The calculations indicate there exits a pressure-induced structural phase transition from the ambient zinc-blende (B3) phase to rock-salt (B1) phase above 24.5 GPa. The first reported Cmcm phase will appear above 87.9 GPa. The calculated valence charge density and density of states show that there exists a charge transfer toward the cation B atom. The pressure dependence of the elastic constants ( C ij ) and the aggregated elastic modulus have been investigated. Moveover, the variations of Debye temperature, the longitudinal and transverse elastic wave velocities with pressure have been presented.

Density functional study of stability of high pressure phases in InP and InAs crystals

The sequence of high pressure phases in InP and InAs crystals was studied by density functional calculations. The transition from ambient pressure zinc-blende to rocksalt (NaCl) phase is of the first order and transitions pressures for crystals are in agreement with experiments. The high pressure phase transition from cubic NaCl phase to orthorhombic Cmcm phase in InP and InAs was studied by monitoring the pressure dependence of transversal acoustic (TA) phonon frequencies. Lattice dynamics calculations reveal that the rocksalt structure is unstable with respect to TA mode at a single point on the Brillouin zone boundary. The observed pressures of the phase transitions in structural measurements were reproduced more accurately then in previous calculations.

Structural, electronic and optical properties of high pressure stable phases of ZnTe

Abstract We have performed full potential linear augmented plane wave calculations to investigate the pressure induced phase transition in ZnTe. Total energies of three phases (zinc-blende, cinnabar and Cmcm) are calculated using density functional theory formalism under generalized gradient approximation and Engel–Vosko generalized gradient approximation for the exchange correlation potential approximation. The pressure stability corresponding to zinc-blende, cinnabar and Cmcm phases of ZnTe are computed. We find that cinnabar phase could be formed as a metastable phase by releasing pressure from the high pressure Cmcm phase. The obtained structural, electronic and optical results are compared with previous calculations and available experimental data. Overall good agreement is found.

FIRST-PRINCIPLES STUDY OF THE STRUCTURAL, ELECTRONIC AND OPTICAL PROPERTIES OF MgSiO3 AT HIGH PRESSURE

The high-pressure behavior of perovskite ( MgSiO 3) is studied based on density functional simulations within generalized gradient approximation (GGA). All calculations are performed by using the linear augmented plane waves plus local orbital (LAPW+lo) method to solve the scalar-relativistic Kohn-Sham equations. The static calculations predict a perovskite (pnma phase) — post-perovskite (Cmcm phase) transition occurring at 86 gigapascals (GPa). The similar bulk modulus values, differing only 3 GPa, are given by using three kinds of equation of states. The electronic structure and optical properties of MgSiO 3 at phase transition pressure are also discussed.

Hall-effect and resistivity measurements in CdTe and ZnTe at high pressure: Electronic structure of impurities in the zinc-blende phase and the semimetallic or metallic character of the high-pressure phases

We carried out high-pressure resistivity and Hall-effect measurements in single crystals of CdTe and ZnTe up to 12 GPa. Slight changes of transport parameters in the zincblende phase of CdTe are consitent with the shallow character of donor impurities. Drastic changes in all the transport parameters of CdTe were found around 4 GPa, i.e. close to the onset of the cinnabar to rock-salt transition. In particular, the carrier concentration increases by more than five orders of magnitude. Additionally, an abrupt decrease of the resistivity was detected around 10 GPa. These results are discussed in comparison with optical, thermoelectric, and x-ray diffraction experiments. The metallic character of the Cmcm phase of CdTe is confirmed and a semi-metallic character is determined for the rock-salt phase. In zincblende ZnTe, the increase of the hole concentration by more than two orders of magnitude is proposed to be due to a deep-to-shallow transformation of the acceptor levels. Between 9 and 11 GPa, transport parameters are consistent with the semiconducting character of cinnabar ZnTe. A two orders of magnitude decrease of the resistivity and a carrier-type inversion occurs at 11 GPa, in agreement with the onset of the transition to the Cmcm phase of ZnTe. A metallic character for this phase is deduced.

High pressure induced phase transition in sulfur doped indium phosphide: An angular-dispersive X-ray diffraction and Raman study

The high pressure induced phase transitions in sulfur doped indium phosphide (InP:S) at ambient temperature has been investigated using angular-dispersive X-ray diffraction (ADXRD) and Raman scattering under high pressure up to around 44.6 and 37.4 GPa, respectively. In situ ADXRD measurements found that the transition of InP:S to a rock-salt phase began at 10.4 GPa and completed at 13.3 GPa with a 15.7% volume decrease. Another transition to the orthorhombic structure with space group Cmcm (the Cmcm phase) was found to occur at 35.8 GPa with a 4.1% volume decrease. The fitting of volume compression data to the third-order Birch–Murnaghan equation of state yielded that the zero-pressure isothermal bulk moduli ( B 0 ) and the first-pressure derivatives ( B 0 ′ ) were 74 GPa and 3.9 for the zinc-blende phase, respectively. The rock-salt to Cmcm phase transition pressure increases relative to that of undoped InP of 32 GPa, which may be attributable to the increase of the ionicity of InP:S by S doping favorable to the ionic rock salt structure. In situ Raman measurements have similar findings.

Electron Transport Property of CdTe under High Pressure andModerate Temperature by In-Situ Resistivity Measurement in Diamond AnvilCell

In situ resistivity measurement has been performed to investigate theelectron transport property of powered CdTe under high pressure andmoderate temperature in a designed diamond anvil cell. Several abnormalresistivity changes can be found at room temperature when the pressureincreases from ambient to 33 GPa. The abnormal resistivity changes atabout 3.8 GPa and 10 GPa are caused by the structural phase transitionsto the rock-salt phase and to the Cmcm phase, respectively. The otherabnormal resistivity changes at about 6.5 GPa, 15.5 GPa, 22.2 GPa andabout 30 GPa never observed before are due to the electronic phasetransitions of CdTe. The origin of the abnormal change occurred atabout 6.5 GPa is discussed. The temperature dependence of theresistivity of CdTe shows its semiconducting behaviour at least before11.3 GPa.

Thermoelectric properties and phase transitions of II–VI semiconductors at high pressure

AbstractThe high‐pressure phase transitions in II–VI semiconductors (HgTe, HgSe, HgS, HgO, CdTe, CdSe, CdS, ZnTe, ZnSe, ZnS, ZnO) are analyzed in light of recent findings. A contribution is emphasized of the thermoelectric power (Seebeck effect) technique to registration of new electronic phases and to prediction of their related structural modifications. A novel thermopower study of ZnX (X–Te, Se, S) revealed a difference between the high‐pressure NaCl phases of ZnSe and ZnS and the orthorhombic Cmcm phase of ZnTe. Mechanisms of the pressure‐induced transitions in ZnX and other II–VI compounds are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ground state structures in the polonium based II–VI compounds

We have performed ab-initio self-consistent calculations using the full-potential linear augmented plane-wave method to investigate the structural and the electronic properties of the less known II–VI compounds: ZnPo, CdPo, and HgPo. Total energy calculations of the cubic zinc-blende, wurtzite, rocksalt, cesuim chloride, orthorhombic Cmcm, and tetragonal PbO phases are investigated. Ground state parameters are computed, and compared with available theoretical and experimental works. The zinc-blende structure is found to be the ground state phase of ZnPo and CdPo, while HgPo prefers the tetragonal PbO structure. The calculated band structure of II-Po shows features that differ considerably from those of typical II–VI semiconductors. In particular we found an inverted band gap, reflecting a semi-metallic character for these compounds.

Hyperfine characterization of SrTi xHf 1−xO 3

In order to study how the Hf replacement by Ti affects the crystalline structure of SrHfO 3 pure perovskite, X-ray diffraction (XRD) studies and hyperfine characterizations of SrHf 1− x Ti x O 3 for the compositions x=0.25, 0.50 and 0.75 are reported. The structure at room temperature (RT) was determined by XRD. Hyperfine electric quadrupole interaction at Ta probes was determined by perturbed angular correlation spectroscopy. For every sample, spin precession curves were measured from RT to 1000 °C, in 50 °C steps. From the data fit it was determined that for x=0.25 the compound is orthorhombic (Pnma) at RT and undergoes one first-order and one second-order phase transitions at 550 and 750 °C, respectively. For x=0.50 the compound is in the orthorhombic (Cmcm) phase at RT and undergoes a second-order phase transition at about 250 °C. For x=0.75, the compound is cubic ( Pm 3 ¯ m ) from RT on. Also, comparison with other perovskite-type compound studies is performed.

Cmcmphase of GeS at high pressure

We study the pressure-induced phase transition of the layered GeS structure using a constant-pressure ab initio technique. For the first time, we predict a gradual phase transition to a Cmcm structure with the application of hydrostatic pressure. The high-pressure phase is still a layered structure, consisting of rocksaltlike bilayers, but it has an unusual fivefold coordination. The transition is due to the significant decrease of second neighbor distances. We also find the metallization of GeS prior to transforming into the Cmcm phase.

How do electronic properties of conventional III–V semiconductors hold for the III–V boron bismuth BBi compound?

AbstractWe have performed ab‐initio self‐consistent calculations using the full potential linear augmented plane wave method to investigate the structural and the electronic properties of the boron bismuth III–V compound BBi. Our calculations provide the first available information about the structural and electronic ground‐state properties of BBi. Total energy calculations of the cubic zinc‐blende, wurtzite, rock‐salt, cesium chloride and orthorhombic Cmcm phases are made. The zinc‐blende structure is found to be the ground‐state phase of BBi; within the generalized gradient approximation (local density approximation), we found a lattice constant of 5.529 Å (5.416 Å) and a bulk modulus of 72.20 GPa (86.27 GPa). We found that, contrary to other boron compounds, the band gap of BBi is direct at the Γ point. The relativistic contraction of the 6s orbital of Bi has a strong influence on the bands and bonds of BBi. Consequently, the electronic properties of BBi are shown to differ considerably from those of common group III–V semiconductors (e.g. GaAs); in particular, we found an unusually strong p–p mixing of the valence‐band maximum relative to most of the other III–V compounds. Furthermore, the calculated valence charge density shows an anomalous behavior, characterized by a charge transfer towards the ‘cation' B atom, further illustrating the rich behavior of boron bismuth compounds. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Space group and crystal structure of the Perovskite CaTiO 3 from 296 to 1720 K

The structural phase transitions of the calcium titanate perovskite CaTiO 3 were investigated by the Rietveld analysis of high-temperature neutron and X-ray powder diffraction data in the temperature range of 296–1720 K. The present work demonstrates that the compound exhibits two reversible phase transitions of orthorhombic Pbnm-tetragonal I4/ mcm and of I4/ mcm-cubic Pm 3 ¯ m at 1498±25 K and 1634±13 K, respectively. No evidence of Cmcm phase is observed between the Pbnm and I4/ mcm structures. The lattice parameters discontinuously change at the Pbnm– I4/ mcm transition point, while a continuous change is observed for the I 4 / mcm – Pm 3 ¯ m transformation. These results indicate that the Pbnm– I4/ mcm transition is of first order and that the I 4 / mcm – Pm 3 ¯ m transformation is of either second or higher order.

EXAFS study of the local structure of InAs up to 80 GPa

The present paper concerns x-ray absorption spectroscopy experiments at the As K-edge onInAs up to 80 GPa. The aim of this study is to obtain quantitative information on the localenvironment of As in the post-NaCl phases of InAs. These measurements confirm theabsence of the CsCl phase, and the analysis of the fine structure of the x-ray absorptionallows us to characterize the local structure of the high pressure phases of InAsfrom their onset of appearance. While the Cmcm phase is locally very similar tothe NaCl structure, having a first coordination shell of six indium atoms, above31 GPa an increasing number of As atoms are detected within a distance of 3.16 Å.

High-pressure phase diagram ofZnSexTe1−xalloys

We have performed high-pressure energy dispersive x-ray diffraction experiments in $\mathrm{Zn}{\mathrm{Se}}_{x}{\mathrm{Te}}_{1\ensuremath{-}x}$ alloys ($x=0$, 0.1, 0.2, 0.55, 0.81, 0.93, 0.99, and 1.0) in order to establish their pressure-composition phase diagram, and characterize the equation of state of all the involved phases. We conclude that the electronic energy-volume dependence is similar in all compounds. The differences observed in the stability range of the cinnabar and Cmcm phases should then be discussed in terms of other factors, as the presence of energy barriers (cinnabar) or dynamical instabilities (Cmcm). We also argue that the cinnabar phase found is similar for all compositions. Finally we discuss the different compressibility observed in the Cmcm and rocksalt phases.

High‐pressure stability and structural properties of CdS and CdSe

AbstractThe structural phase transformations of CdS and CdSe under high pressure are studied by using the local approximation to the density functional theory, and the one‐electron equations are solved by means of the full‐potential linear muffin‐tin‐orbital method FP‐LMTO. CdS and CdSe are found to have nearly similar structural systematics under high pressure. In CdS, the Pmmn phase is predicted after the rocksalt structure, and in CdSe the Cmcm structure is thermodynamically stable after the rocksalt structure. We also find a thermodynamic stability range for the CsCl phase of CdSe. The structural properties of the zincblende, wurtzite, rocksalt, Pmmn, and Cmcm phases are presented. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

STRUCTURAL CHARACTERIZATION OF THE CINNABAR PHASE IN ZnSexTe1-x ALLOYS

We report energy dispersive x-ray diffraction experiments carried out in the Te rich side of ZnSexTe1-x alloys with the aim of characterizing the cinnabar phase. In the samples studied (x =0.55, 0.2, 0.1 and 0.0) the cinnabar phase is observed both in the upstroke and downstroke, between either the zincblende and rocksalt phases (x =0.55, 0.2), or between the zincblende and Cmcm phases (x =0.1, 0.0). In ZnSe0.2Te0.8 the rocksalt phase is transformed to the Cmcm phase if additional pressure is applied. The dependence of the cinnabar's lattice parameters with composition is given by a=4.062 — 0.262x and c =9.434 — 0.575x.

Vibrational properties of ZnTe at high pressures

Raman spectra of ZnTe were measured under hydrostatic pressuresup to 15 GPa at T = 300 K. Results for the frequencies offirst- and second-order Raman features of the zincblende phase(0-9.5 GPa) are used to set up a rigid-ion model of the phonondispersion relations under pressure. Calculated phonon densitiesof states, mode Grüneisen parameters and the thermalexpansion coefficient as a function of pressure are discussed.The effect of pressure on the widths and intensities of Ramanspectral features is considered. Raman spectra of high-pressurephases of ZnTe are reported. These spectra indicate the possibleexistence of a new phase near 13 GPa, intermediate between thecinnabar and orthorhombic (Cmcm) phases of ZnTe.