High-Pressure Raman Study of Zincblende,Cinnabar, and Cmcm Phases Of ZnTe

Raman measurements of ZnTe have been performed at pressures up to 15 GPa. Frequencies, line widths, and intensities of first- and second-order Raman features of the zincblende phase (0-9.5 GPa) were studied in detail. In this note, we focus on the Raman spectra of the high-pressure cinnabar and Cmcm phases. In the transition regime from cinnabar to Cmcm (12.2 to 13.7 GPa) the Raman data indicate the possible existence of a new intermediate high-pressure phase.     

闪锌矿结构的PtN:一种不稳定的过渡金属氮化物

基于密度泛函理论的第一性原理的赝势方法，采用局域密度近似和广义梯度近似，研究了闪 锌矿结构的PtN晶体的电子结构和力学稳定性.结果表明具有闪锌矿结构的PtN是一种力学不 稳定的金属氮化物，岩盐矿结构PtN的聚合能略大于闪锌矿结构PtN的聚合能，在适当压力条 件下可以发生从闪锌矿到岩盐矿的相变. 关键词： PtN 电子结构 力学稳定性 第一原理计算     

Trap-recharging waves versus damped,forced charge-density oscillations in hexagonal silicon carbide

The structural parameters and hydrostatic pressure coefficients of CdS_xTe_1-x in the two phases, namely zinc-blende and NaCl as well as the transition pressures from zinc-blende to NaCl structures at various S concentrations are presented. The calculations are performed using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the local density approximation (LDA), and two developed refinements, namely the generalized gradient approximation (GGA) of Perdew et al. for the structural properties and Engel-Vosko for the band structure calculations. Detailed comparisons are made with published experimental and theoretical data and show generally good agreement. The present results regarding the studied quantities for compositions x in the 0–1 range (0 < x < 1) and for the NaCl phase are predictions and may serve as a reference for experimental work.     

X-ray Diffraction Study of Cd 0.8 Zn 0.2 Te Under High Pressure and Temperature

Energy dispersive X-ray diffraction using synchrotron radiation has been used to study phase transformations of Cd 0.8 Zn 0.2 Te under high pressure and temperature. We confirm the presence of a cinnabar phase between the zinc-blende and rock-salt phases. But contrary to the results in CdTe, this intermediate phase is found to be stable only on pressure decrease and in a narrower pressure and temperature range. Single-phase cinnabar patterns are obtained only at 300 and 373 K. At 673 K and above, even on pressure decrease, no evidence of the cinnabar phase is found. In this temperature range, a phase segregation phenomenon is observed in the zinc-blende phase during the zinc-blende transition in both upstroke and downstroke, and the retrieved sample at ambient conditions presents a double pattern corresponding to two different Zn contents.     

Phase Transition and Phonon Spectrum of Zinc-Blende Structure ZnX （X = S,Se, Te）

Calculations have been performed to investigate the pressure-induced solid-solid phase transitions and the mechanical stability for three zinc-blende II-VI semiconductor compounds： ZnS, ZnSe, ZnTe by ab initio plane-wave pseudopotential density functional theory （DFT）. Using the generalized gradient approximation （GGA） for exchange and correlation in the scheme of Perdew-Wang 1991 （P Wgl ）, the ground state properties and equation of state are obtained, which are well consistent with the experimental data available and other calculations. On the basis of the forth-order Birch-Murnaghan equation of states, the transition pressures Pt are determined through the analysis of enthalpy variation with pressure. A linear-response approach is used to calculate the frequencies of the phonon dispersion. Finally, by the calculations of phonon frequencies, some thermodynamic properties such as the vibrational contribution to the Helmholtz free energy （F）, enthedpy （H）, entropy （S）, and the heat capacity （Cv ） are also successfully obtained.     

Optical Properties of High Pressure Phases in ZnTe 1− x Se x

We report on the optical properties of high pressure semiconducting phases in ZnTe 1 m x Se x . In the Te rich side, the cinnabar phase is observed in the upstroke between typically 9.5 and 12.5 GPa with a pressure interval of existence that decreases with increasing the Se content. In most studied samples, the indirect absorption edge could be determined, with values of the bandgap increasing with the Se content and ranging from 1.2 to 1.7 eV. In the downstroke, the cinnabar phase is observed in the whole composition range but its bandgap can not be unambiguously determined in the Se-rich side, as it coexists with rocksalt or zincblende phases. The indirect semiconducting rocksalt phase is observed in the Se-rich side, with an indirect bandgap of the order of 0.7 eV. Within the experimental errors, the bandgaps of both the cinnabar and NaCl phases are pressure insensitive, in agreement with first-principles pseudopotential band structure calculations, that predict very low pressure coefficients for both indirect transitions.     

Vibrational properties and phase transitions in II-VI materials: lattice dynamics, ab initio studies and inelastic neutron scattering measurements

Inelastic neutron scattering measurements were carried out to determine the phonon density of states of ZnSe and interpreted with lattice dynamical computations (ab initio as well as a potential model). Calculations are also reported for other II-VI compounds, ZnTe and ZnS. Vibrational (phonon spectra and Grüneisen parameters), and thermal (negative thermal expansion and non-Debye specific heat) properties have been calculated and found to be in good agreement with available experimental data. This model has been further employed to study the pressure-induced solid-solid phase transitions exhibited by these compounds and the results have been compared with experimental data. Total energy calculations for zincblende and SC16 phases of ZnSe were carried out employing the pseudopotential approach under the local density approximation (LDA) as well as the generalized gradient approximation (GGA). The density functional perturbation theory is applied to study the vibrational properties of the zincblende and SC16 phases of ZnSe. An investigation of the pressure dependence of the phonon frequencies shows that the existence of the (experimentally undetected) SC16 phase as a thermodynamically stable high pressure phase is impeded due to dynamical instabilities. A detailed investigation of the polarization of phonons of different energies for the various phases of these compounds indicates that in the case of the zincblende phase the low energy modes are librational, while in the rocksalt phase the low energy modes are bending modes. Further, in ZnTe the low energy bending modes display a larger amplitude of bending than that in ZnSe and ZnS.     

Composition dependence of the optical properties and band structure of the zinc-blende ZnS1-xOx: a first principles study

We present first principles calculations of structural, electronic and optical properties of ZnS_1?xO_x in the zinc-blende phase. We employ the full potential linearized augmented plane wave method within the density functional theory in the generalized gradient approximation and Engel–Vosko generalized gradient approximation. Features such as the lattice constant, the bulk modulus and its pressure derivative are reported. The agreement between our calculated results and available experimental and theoretical data is generally good. Direct and indirect energy band gaps as a function of the oxygen composition in the material of interest are presented and discussed. The material under investigation is found to remain a direct band gap semiconductor over all the alloy composition range (0–1). Furthermore, the optical properties such as the dielectric function, the refractive index, the reflectivity and the electron loss energy have also been reported and analysed.     

Phase transition, elastic, thermodynamic properties of zinc-blend BeSe from first-principles

The structural phase transition, elastic, thermodynamics properties of BeSe in zinc-blende were investigated by performing first-principles calculations within generalized gradient approximation. The phase transition pressure P_t between the B3 phase and the B8 phase of BeSe was determined. The pressure dependencies of elastic constants, shear modulus, Young's modulus, and Poisson's ratio of BeSe are calculated. The thermodynamic properties of the zinc-blende structure BeSe are calculated by using the quasi-harmonic Debye model. The pressure and temperature dependencies of the heat capacity and the thermal expansion coefficient, as well as the Grüneisen parameter are investigated systematically in the ranges of 0–50 GPa and 0–1200 K.     

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.     

Pressure effects on the thermodynamic and mechanical properties of zinc-blende ZnTe compound

In this paper, the moment method in statistical mechanics has been employed to study the pressure effects on thermodynamic and mechanical properties of zinc-blende zinc telluride using many-body potential. We have derived the analytical expressions of the pressure-dependent lattice parameter, volume compression as well as mean-square displacement of zinc-blende type compound. Numerical calculations performed for ZnTe compound up to 12 GPa are found to be in good and reasonable agreement with available experimental data as well as with previous theoretical studies. These results have been used to evaluate the bulk modulus and its first pressure derivative of ZnTe. The present moment method has taken into account the quantum zero-point vibrations at low temperature and the higher-order anharmonic terms in the atomic displacements. This research shows the advantage of moment method on extensively studying thermo-mechanical properties of materials under high pressures.     

Ab-initio calculation of stability and structural properties of cadmium chalcogenides CdS, CdSe, and CdTe under high pressure

The structural phase transformations of Cd chalcogenide compounds (CdS, CdSe, and CdTe) under high pressure are studied 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. CdS, CdSe, and CdTe are found to have nearly similar behaviour of the structural properties under high pressure. In the CdS compound, thePmmn phase is predicted to be thermodynamically stable after the rock-salt structure, and theCmcm structure is found to be thermodynamically stable before the rock-salt structure in both CdSe and CdTe. The resulting structural properties of the zinc-blende, wurtzite, cinnabar, rock-salt,Pmmn, CsCl,Cmcm, and β-Sn phases for these considered compounds are found to be in agreement with the existing experimental data.     

Density functional study of optical properties of beryllium chalcogenides compounds in nickel arsenide B8 structure

The structural, electronic and optical properties of beryllium chalcogenides BeS, BeSe and BeTe using the full-potential linear augmented plane wave (FP-LAPW) method are investigated. The exchange-correlation energy within the local density approximation (LDA) and the generalized gradient approximation (GGA) are described. The Engel-Vosko (EVGGA) formalism is applied for electronic and optical properties. The structural parameters of our model and the transition pressure from zinc-blende (B3) to the NiAs (B8) phase are confirmed. It is found that these compounds have indirect band gaps except for BeTe in NiAs (B8) phase. The results of reflectivity, refractive index and optical dielectric functions of Be compounds are investigated. An agreement is found between our results and those of other theoretical calculations and the experimental data.     

Phase stability and electronic properties of silver halides

In this work, we study the phase stability and electronic properties of silver halides ( AgBr, AgCl and AgI) using the full-potential linearized augmented plane wave method within the density functional theory. In this approach, the Wu–Cohen generalized gradient approximation was used for the exchange–correlation potential. Moreover, the modified Becke–Johnson approximation was also used for band-structure calculations. Various structural phase transitions were considered here in order to confirm the most stable structure and to predict the phase transition under hydrostatic pressure. In addition, we have studied the band structures of the stable phases of these compounds which reveal that the three compounds exhibit semiconducting behavior. The results obtained are compared with other calculations and experimental measurements.     

Half-metallic ferromagnetism of zinc-blende CrS and CrP: a first-principles pseudopotential study

The electronic structure and the ferromagnetism of CrS and CrP in the zinc-blende (ZB) phase are investigated by spin-polarized calculations with first-principles plane-wave pseudopotential method within the generalized gradient approximation for the exchange-correlation potential. From the analysis of the spin-dependent density of states, band structure and magnetic moment, we predict that ZB CrS and CrP at their respective equilibrium lattice constant are half-metallic ferromagnets with a magnetic moment of 4.00 and 3.00μ_B per formula unit, respectively. We also find that the ZB CrS maintains half-metallic ferromagnetism up to 3% compression of lattice constant while the half-metallic ferromagnetism for ZB CrP exists only near its equilibrium lattice constant.     

Ab-initio study of electronic and optical properties of InN in wurtzite and cubic phases

We have performed systematic first principle calculations for the electronic and optical properties of a narrow band gap semiconductor InN in cubic and wurtzite phases by ‘state-of-the-art’ DFT calculations within generalized gradient approximation (GGA) and Engel-Vosko's corrected generalized gradient approximation (EVGGA) using full potential linear augmented plane wave (FPLAPW) method as implemented in WIEN2k code. The total energy for the wurtzite phase of InN was found to be smaller by 0.0184 Ry/molecule by cubic phase which confirms the greater stability of the wurtzite structure than the cubic one. Band structure, effective masses, density of states, valence charge densities, and dielectric functions are computed and presented in detail. The critical points are extracted out of calculated dielectric function, compared with available measured data and are explained in terms of transitions occurred in the band structure along different symmetry and antisymmetry lines. The valence band maxima and conduction band minima are strongly dominated by N-2p states and located at the Γ-symmetrical line which predicts its direct band gap nature in both phases.     

Electronic properties and stability of ZnO from computational study

The ground-state properties of ZnO in the rock-salt (B1), CsCl (B2), zinc-blende (B3), wurtzite (B4), cinnabar, cmcm, d-β-tin, NiAs, Immm, and Imm2 structures were investigated using an accurate first-principles total-energy calculations based on the full-potential augmented plane-wave plus local orbitals (APW+lo) method. The local density approximation was used for the exchange and correlation energy density functional. The ground state properties such as lattice parameter, bulk modulus and its pressure derivative as well as the structural phase stability were calculated and compared to the available experimental data and previous theoretical works.     

Electronic and optical properties of high pressure stable phases of ZnS: Comparison of FPLAPW and PW-PP results

A theoretical study of the structural phase transformation of ZnS under high pressure has been performed using first principle plane wave pseudopotential (PW-PP) and full potential linear augmented plane wave method (FPLAPW) calculation in which Zn-3d states are treated as valence states. In both methods, we have used a generalized gradiant approximation for the study of phase transformation and structural parameters. The calculated difference in lattice constants (Δα₀) by PW-PP and FPLAPW methods for zinc-blende, cinnabar and rocksalt structures is equal to 0.003, 0.01 and 0.001 Å respectively. There is a very good agreement between the results of PW-PP and FPLAPW calculations that shows soundness of our choice of pseudopotential. The calculated transition pressure for zinc-blende → rocksalt is in agreement with available measured data. We present calculations of the optical properties for three phases of ZnS. The band gap of different phases of ZnS decreases in order of zinc-blende → cinnabar → rocksalt mainly due to red shift of Zn-s states in the lowest conduction band. Besides, the optical band gap decreases from 2.84 eV (direct) to 0.188 eV (indirect). The shift of calculated complex dielectric function ε₂(ω) for zinc-blende → cinnabar → rocksalt is also discussed in details of optical transition that occurred in different phases.     

Electronic and optical properties of BAs under pressure

The electronic and optical properties of boron arsenide (BAs) in the zinc-blende (ZB) and rock-salt (RS) phases have been studied by the density functional theory (DFT) method based on the generalized gradient approximation (GGA). Using the enthalpy-pressure data, the structural phase transition from ZB to RS is observed at 141 GPa. Our calculated electronic properties show that ZB-BAs is a semiconductor, whereas RS-BAs is a semi-metal. Calculations of the dielectric function and absorption coefficient have been performed for the energy range 0-30 eV. The dependence of pressure on band structure and optical spectra is also investigated. The results are compared with available theoretical and experimental data.     

Calculated optical properties of GaX (X=P,As, Sb) under hydrostatic pressure

The hydrostatic pressure dependence of the principal energy gaps and of the optical properties of GaX (X = P, As and Sb) has been calculated using the full potential-linearized augmented plane wave (FP-LAPW) method. The generalized gradient approximation (GGA) for the exchange and correlation potential is applied. Also, we have used the Engel–Vosko GGA formalism, which optimizes the corresponding potential for band-structure and the optical properties calculations. Structural properties such as equilibrium lattice constants, the bulk modulus, and its pressure derivatives were calculated for GaP, GaAs, and GaSb in the zinc-blende structure (ZB). We have found that the results of the structural properties calculations are in agreement with those of ab initio and experimental data. In general, the pressure dependence of the principal energy gaps is compared to other values. The same is for the pressure coefficient. However, for the same structure, the comparison of our results with those of experimental and theoretical calculations shows good agreement. On the other hand, the effect of the applied pressure is clearly seen in the optical properties especially near the energy transition regions.