Structural phase transition (zincblende-rocksalt) and elastic properties in AlY (Y=N, P and As) compounds: Pressure-induced effects

The present paper addresses the pressure-induced structural aspects of ZnS-type (B3) to NaCl-type (B1) structure in AlY (Y=N, P, As). An effective-interionic interaction potential (EIoIP) with long-range Coulomb and three-body interactions and the Hafemeister-and-Flygare-type short-range overlap repulsion extended up to the second-neighbour ions and the van der Waals (vdW) interaction is developed. Emphasis has been given on evaluating the vdW coefficients by the Slater-Kirkwood variational method, as both the ions are polarizable. The lattice model calculations have revealed reasonably good agreement with the available experimental data on the phase-transition pressures (P_t=16, 14, 7.5 GPa) and the elastic properties of AlY (Y=N, P, As). The equation of state curves (plotted between V(P)/V(0) and pressure) for both the B3 and B1 structures obtained are in fairly good agreement with the experimental results. The calculated values of the volume collapses [ΔV(P)/V(0)] are also close to their observed data. Further, the variations of the second-order elastic constants with pressure follow a systematic trend that is almost identical to that exhibited by the observed data measured for other semiconducting compounds with B3→B1 structural phase transitions.     

Pressure induced B3-B1 structural phase transition and elastic properties of monopnictides InX (X = N,P, As)

An effective interionic interaction potential is developed to discuss the pressure induced structural phase transformation and mechanical properties of InX (X = N, P, As) semiconducting compounds. The effective interionic potential consists of the long-range Coulomb and three-body interactions and the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbour ions and the van der Waals interaction. The present calculations have revealed reasonably good agreement with the available experimental data on the phase transition pressures (P_t = 11.5, 10, 7.5 GPa) and the elastic properties of InX (X = N, P, As). The equation of state curves (plotted between V (P)/V(0) and pressure) for both the structures zincblende (B3) and rocksalt (B1) structures obtained by us are in fairly good agreement with the experimental results. The calculated values of the volume collapses [ΔV(P)/V(0)] are also closer to their observed data.     

Structural phase transition and elastic properties of ZnSe at high pressure

We have evolved an effective interionic interaction potential to investigate the pressure-induced phase transitions from zinc blende (B3) to rock salt (B1) structure in II-VI [ZnSe] semiconductors. The elastic constants, including the long-range Coulomb and van der Waals (vdW) interactions and the short-range repulsive interaction of up to second-neighbor ions within the Hafemeister and Flygare approach, are deduced. Keeping in mind that both of the ions are polarisable, we employed the Slater-Kirkwood variational method to estimate the vdW coefficients. The estimated value of the phase transition pressure (P _t ) is higher than in the reported data, and the magnitude of the discontinuity in volume at the transition pressure is consistent with that data. The major volume discontinuity in the pressure-volume phase diagram identifies the structural phase transition from zinc blende to rock salt structure.

The variation of second-order elastic constants with pressure resembles that observed in some binary semiconductors. It is inferred that the vdW interaction is effective in obtaining the thermodynamic parameters such as the Debye temperature, the Gruneisen parameter, the thermal expansion coefficient and the compressibility. However, the inconsistency between the thermodynamic parameters as obtained from present model calculations and their experimental values is attributed to the fact that we have derived our expressions by assuming the overlap repulsion to be significant only up to the nearest second-neighbor ions, as well as neglecting thermal effects. It is thus argued that full analysis of the many physical interactions that are essential to binary semiconductors will lead to a consistent explanation of the structural and elastic properties of II–VI semiconductors.     

Pressure dependent elastic properties of diluted magnetic semiconductors: Hg1−xMnxS (x=0.02 and 0.07)

A theoretical study of the elastic properties in diluted magnetic semiconductors Hg_1−xMn_xS (x=0.02 and 0.07) using an effective interionic interaction potential (EIoIP) in which long-range Coulomb interactions, charge transfer mechanism (three body interaction) and the Hafemeister and Flygare type short-range overlap repulsion extending up to the second neighbor ions and the van der Waals (vdW) interaction is considered. Particular attention is devoted to evaluate Poisson's ratio ν, the ratio R_S/B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and thermodynamic property as Debye temperature is calculated. By analyzing Poisson's ratio ν and the ratio R_S/B we conclude that Hg_1−xMn_xS is brittle in zinc blende (B3). To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Hg_1−xMn_xS compounds and still awaits experimental confirmations.     

Pressure dependent mechanical and thermodynamical properties of Hg<Subscript>0.91</Subscript>Mn<Subscript>0.09</Subscript>Te semiconductor

The mechanical, thermodynamical and elastic properties of Hg_0.91Mn_0.09Te compound are calculated by formulating an effective interionic interaction potential. This potential consists of the long-range Coulomb, three body force parameter, the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction. The estimated values of phase transition pressure have revealed reasonably good agreement with the available experimental data on the phase transition pressure P _t = 11.5 GPa and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zincblende (B3) to rock salt (B1) structure. Later on, the Poisson’s ratio ν, the ratio R _S/B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and Debye temperature as functions of pressure is calculated. From Poisson’s ratio and the ratio R _S/B it is inferred that Hg_0.91Mn_0.09Te is brittle in nature in both B3 phase and B1 phase. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Hg_0.91Mn_0.09Te compounds and still awaits experimental confirmations.     

High pressure structural phase transition and elastic properties of Ga<Subscript>1-x</Subscript>In<Subscript>x</Subscript>As semiconducting compounds

The present paper addresses the high-pressure phase transformation and mechanical properties of Ga_1-xIn_xAs (x = 0.25, 0.5 and 0.75) by formulating an effective interionic interaction potential. This potential consists of the long-range Coulomb and charge transfer caused by the deformation of the electron shells of the overlapping ions and the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction. The estimated values of phase transition pressure and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zinc blende (B3) to rock salt (B1). The equation of state curves plotted between V (P)/ V (0) and pressure are for both the zincblende (B3) and rocksalt (B1) structures. Further, the variations of the second and third order elastic constants with pressure have followed a systematic trend, which are almost identical to those exhibited by the observed data measured for other compounds of this family.     

Pressure induced phase transition in ZnS

The phase transition of ZnS from the zincblende (ZB) structure to the rocksalt (RS) structure is investigated by the ab initio plane-wave pseudopotential density functional theory method. It is found that the pressures for transition from the ZB structure to the RS structure are 17.5 GPa from total energy-volume data and 15.4 GPa from equal enthalpies, consistent with the experimental data. From the high pressure elastic constants obtained, we find that the ZB structure ZnS is unstable when the applied pressure is larger than 17 GPa. Moreover, the dependence of the normalized primitive cell volume V/V₀ on pressure P can also be successfully obtained.     

A theoretical study of structural, elastic and thermal properties of heavy lanthanide monoantimonides

The structural, elastic and thermal properties of three heavy monoantimonides of holmium, erbium and thulium (LnSb, Ln=Ho, Er and Tm) have been investigated theoretically by using an interionic potential theory consisting of long-range Coulomb, short-range repulsive and van der Waal’s (vdW) interactions. These compounds exhibit first-order crystallographic phase transition from their initial NaCl-type structure to CsCl-type structure at pressures 27, 33.2 and 29.8 GPa for HoSb, ErSb and TmSb, respectively. The values of elastic constants and Debye temperatures as a function of pressure are also reported. The elastic properties such as Young modulus (E), Shear modulus (G), Poisson ratio (υ) and anisotropic ratio (A) in an NaCl-type structure are also predicted.     

Structural and elastic properties of barium chalcogenides (BaX,X=O,Se, Te) under high pressure

In the present paper we have investigated the high-pressure, structural phase transition of Barium chalcogenides (BaO, BaSe and BaTe) using a three-body interaction potential (MTBIP) approach, modified by incorporating covalency effects. Phase transition pressures are associated with a sudden collapse in volume. The phase transition pressures and associated volume collapses obtained from TBIP show a reasonably good agreement with experimental data. Here, the transition pressure, NaCl-CsCl structure increases with decreasing cation-to-anion radii ratio. In addition, the elastic constants and their combinations with pressure are also reported. It is found that TBP incorporating a covalency effect may predict the phase transition pressure, the elastic constants and the pressure derivatives of other chalcogenides as well.      

Structural, high pressure and elastic properties of transition metal monocarbides: A FP-LAPW study

The structural, elastic and thermal properties of four transition metal monocarbides ScC, YC (group III), VC and NbC (group V) have been investigated using full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA) both at ambient and high pressure. We predict a B₁ to B₂ structural phase transition at 127.8 and 80.4 GPa for ScC and YC along with the volume collapse percentage of 7.6 and 8.4%, respectively. No phase transition is observed in case of VC and NbC up to pressure 400 and 360 GPa, respectively. The ground state properties such as equilibrium lattice constant (a₀), bulk modulus (B) and its pressure derivative (B′) are determined and compared with available data. We have computed the elastic moduli and Debye temperature and report their variation as a function of pressure.     

Pressure-induced structural phase transition and elastic properties of rare earth Pr chalcogenides and pnictides

Pressure-induced structural aspects and elastic properties of NaCl-type (B1) to CsCl-type (B2) structure in praseodymium chalcogenides and pnictides are presented. Ground-state properties are numerically computed by considering long-range Coulomb interactions, Hafemeister and Flygare type short-range overlap repulsion, and van der Waals interaction in the interionic potential. From the elastic constants, Poisson's ratio ν, the ratio R_G/B of G (shear modulus) over B (bulk modulus), anisotropy parameter, shear and Young's moduli, Lamé's constant, Kleinman parameter, elastic wave velocity and thermodynamical property such as Debye temperature are calculated. Poisson's ratio ν and the ratio R_G/B indicate that PrX and PrY are brittle in B1 phase and ductile in B2 phase. To our knowledge, this is the first quantitative theoretical prediction of the ductile (brittle) nature of praseodymium chalcogenides and pnictides and still awaits experimental confirmation.     

Optical,physical and structural studies of vanadium doped P2O5–BaO–Li2O glasses

Glasses in the compositions (Li₂O)₂₅–(BaO)₂₅–(P₂O₅)_50−x–(V₂O₅)_x (with x=0.5,1.0,1.5,2.0,2.5, and 3.0 mol%) have been prepared by the conventional melt quenching technique. X-ray powder diffractrogram show broad peaks which conforms glassy nature of the sample. Differential scanning calorimetry (DSC) thermograms show characteristic glass transition temperature (T_g) and it increases with increasing substitution of V₂O₅ for P₂O₅. The measured physical parameters like density, refractive index, ionic concentration and electronic polarizability are found to vary linearly with increasing x. Infrared spectra exhibits few bands, which are attributed to (P=O)_AS, (P=O)_S, (V=O), (P–O–P)_AS,P–O–V, (P–O–P)_AS and O–P–O vibrations. The optical absorption spectra of VO²⁺ ions in these glasses show three bands and are assigned to the ²B₂→ ²E,²B₂→ ²B₁ and ²B₂→ ²A₁ transitions. Electron paramagnetic resonance spectra of all the glass samples exhibit resonance signals characteristic of VO²⁺ ions. The values of Spin-Hamiltonian parameters indicate that the VO²⁺ ions are present in octahedral sites with tetragonal compression and belong to C_4V symmetry.     

High pressure effect on structural and mechanical properties of some LnO (Ln=Sm, Eu, Yb) compounds

The structural and mechanical properties of LnO (Ln=Sm, Eu, Yb) compounds have been investigated using a modified interionic potential theory, which includes the effect of Coulomb screening. We predicted a structural phase transition from NaCl (B₁)- to CsCl (B₂)-type structure and elastic properties in LnO compounds at very high pressure. The anomalous properties of these compounds have been correlated in terms of the hybridisation of f-electrons of the rare earth ion with conduction band and strong mixing of f-states of lanthanides with the p-orbital of neighbouring chalcogen ion. For EuO, the calculated transition pressure, bulk modulus and lattice parameter are close to the experimental data. The nature of bonds between the ions is predicted by simulating the ion-ion (Ln-Ln and Ln-O) distances at high pressure. The second order elastic constants along with shear modulus and Young's modulus, elastic anisotropy and Poisson's ratio are also presented for these oxides.     

Comparative investigations of the P-V-T relationship of NaCl at high pressure and temperature

Two different potential models of molecular dynamics (MD) simulations have been applied to investigate the pressure-volume-temperature (P-V-T) relationship and lattice parameter of NaCl under high pressure and temperature. The first one is the shell model (SM) potentials in which due to the short-range interaction pairs of ions are moved together as is the case in polarization of a crystal due to the motion of the positive and negative ions, and the second one is the two-body rigid-ion Born-Mayer-Huggins-Fumi-Tosi (BMHFT) potentials with full treatment of long-range Coulomb forces. The P-V relationship at 300 K, T-V relationship at zero pressure, and lattice parameter a, have been obtained and compared with the available experimental data and other theoretical results. Compared with SM potentials, the MD simulation with BMHFT potentials is very successful in reproducing accurately the measured volumes of NaCl. At an extended pressure and temperature ranges, P-V relationship under different isotherms at selected temperatures, T-V relationship under different pressures, and lattice parameter a have also been predicted. The properties of NaCl are summarized in the pressure range 0-30 GPa and the temperature up to 2000 K.     

Ab initio study of phase transition and thermodynamic properties of PtN

The transition phase of PtN from zincblende (ZB) structure to rocksalt (RS) structure is investigated by ab initio plane-wave pseudopotential density functional theory method, and the thermodynamic properties of the ZB and RS structures under high pressure and temperature are obtained through the quasi-harmonic Debye model. The transition phase from the ZB structure to the RS structure occurs at the pressure of 18.2 GPa, which agrees well with other calculated values. Moreover, the dependences of the relative volume V/V₀ on the pressure P, the Debye temperature Θ and heat capacity C_V on the pressure P, together with the heat capacity C_V on the temperature T are also successfully obtained.     

Study of high-pressure phase transition of CeTe and EuTe through three-body interaction potential approach

The high pressure phase transition of lanthanum monotellurides having NaCl-type (B₁) structure have been studied using three-body interaction potential (TBIP) approach. The potential model consists of long-range Coulombic, three-body interaction forces, short-range overlap repulsive forces operative up to next nearest neighbor ions, van der Walls interactions and zero point energy effects. To understand the effect of pressure on elastic constant and their combinations, they have also been studied. The Born stability criterion was also found to be fulfiled in the present study. Our calculated results of phase transitions, volume collapses and elastic behavior of these monotellurides are found to be close to the experimental results. This shows that the inclusion of three-body interaction effects makes the present model suitable for high-pressure studies.     

Transition phase and thermodynamic properties of GaN via first-principles calculations

The transition phase of GaN from zincblende (ZB) structure to rocksalt structure (RS) is investigated by ab initio plane-wave pseudopotential density functional theory method, and the thermodynamic properties of the ZB and RS structures are obtained through the quasi-harmonic Debye model. We find that the transition phase from the ZB structure to the RS structure occurs at the pressure of 42.2 GPa, which is in good agreement with other calculated values. Moreover, the dependences of the relative volume V/V₀ on the pressure P, the Debye temperature Θ and heat capacity C_V on the pressure P, as well as the heat capacity C_V on the temperature T are also successfully obtained.     

Pressure-induced structural phase transition in RbAu

Using the first principle method based on density functional theory, the structural and elastic properties calculations of RbAu have been performed. The results demonstrate that RbAu is stable in the CsCl structure (B2) at ambient pressure, which is in well agreement with the experimental results. And there exists a structural phase transition from CsCl-type structure (B2) to NaTi-type structure (B32) at the transition pressure of approximate 6 GPa. The pressure effects on the elastic properties are discussed and the elastic property calculation indicates elastic instability maybe provide phase transition driving force according to the variations relation of the elastic constant versus pressure.