Phase transition and high pressure behavior of Zirconium and Niobium carbides

We have predicted the phase transition pressure (P _T )and high pressure behavior of Zirconium and Niobium carbide (ZrC, NbC). The high pressure structural phase transitions in ZrC and NbC has been studied by using a two body inter-ionic potential model, which includes the Coulomb screening effect, due to the semi-metallic nature of these compounds. These transition metal carbides have been found to undergo NaCl (B1) to CsCl (B2)-type structural phase transition, at high pressure like other binary systems. We predict such structural transformation in ZrC and NbC at a pressure of 98GPa and 85GPa respectively. We have also predicted second order elastic constant and bulk modulus. The present theoretical work has been compared with the corresponding experimental data and prediction of LAPW and GGA and LDA theories.      

Pressure induced structural phase transition and electronic properties of actinide monophospides: Ab-initio calculations

We have investigated the structural and electronic properties of monophospides of thorium, uranium and neptunium. The total energy as a function of volume is obtained by means of the self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). From the present study with the help of total energy calculations it is found that ThP, UP and NpP are stable in NaCl-type structure at ambient pressure. The structural stability of ThP, UP and NpP changes under the application of pressure. We predict a structural phase transition from NaCl-type (B₁-phase) structure to CsCl-type (B₂-phase) structure for these phospides in the pressure range of 37.0-24.0 GPa (ThP-NpP). We also calculate lattice parameter (a₀), bulk modulus (B₀), band structure and density of states. From energy band diagram it is observed that ThP, UP and NpP exhibit metallic behavior. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.     

Theoretical investigation on structural, magnetic and electronic properties of ferromagnetic GdN under pressure

The tight-binding linear muffin tin orbital (TB-LMTO) method within the local density approximation is used to calculate structural, electronic and magnetic properties of GdN under pressure. Both nonmagnetic (NM) and magnetic calculations are performed. The structural and magnetic stabilities are determined from the total energy calculations. The magnetic to ferromagnetic (FM) transition is not calculated. Magnetically, GdN is stable in the FM state, while its ambient structure is found to be stable in the NaCl-type (B₁) structure. We predict NaCl-type to CsCl-type structure phase transition in GdN at a pressure of 30.4 GPa. In a complete spin of FM GdN the electronic band picture of one spin shows metallic, while the other spin shows its semiconducting behavior, resulting in half-metallic behavior at both ambient and high pressures. We have, therefore, calculated electronic band structures, equilibrium lattice constants, cohesive energies, bulk moduli and magnetic moments for GdN in the B₁ and B₂ phases. The magnetic moment, equilibrium lattice parameter and bulk modulus is calculated to be 6.99 μ_B, 4.935 Å and 192.13 GPa, respectively, which are in good agreement with the experimental results.     

A polymerization-depolymerization model for generation of contractile force during bacterial cell division

We have investigated the pressure-induced phase transition of NiO and other structural properties using three-body potential approach. NiO undergoes phase transition from B1 (rocksalt) to B2 (CsCl) structure associated with a sudden collapse in volume showing first-order phase transition. A theoretical study of high pressure phase transition and elastic behaviour in transition metal compounds using a three-body potential caused by the electron shell deformation of the overlapping ion was carried out. The phase transition pressure and other properties predicted by our model is closer to the phase transition pressure predicted by Eto et al.      

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.     

First-principles study of phase transition and elastic properties of ScSb and YSb compounds

The phase transition of ScSb and YSb from the NaCl-type (B1) structure to the CsCl-type (B2) structure is investigated by the ab initio plane-wave pseudopotential density functional theory method. It is found that the pressures for transition from the B1 structure to the B2 structure obtained from the equal enthalpies are 38.3 and 32.1 GPa for ScSb and YSb, respectively. From the variations of elastic constants with pressure, we find that the B1 phase of ScSb and YSb compounds are unstable when applied pressures are larger than 46.3 and 64.2 GPa, respectively. Moreover, the detailed volume changes during phase transition are analyzed.     

Hydrostatic pressure driven spin,volume and band gap collapses in SmFeO3: a GGA + U study

The crystal structure, magnetic and electronic properties of SmFeO₃ under hydrostatic pressure have been studied by first-principles calculations within the generalized gradient approximation plus Hubbard U (GGA + U). The iso-structural phase transition with spin, volume and band gap collapses can be induced by a large enough hydrostatic pressure. The high-spin (HS) state of Fe³⁺, with the magnetic moment of ~4 μ_B, is retained at low pressure. The spin crossover occurs at a transition pressure (~68 GPa) with the magnetic moment of Fe³⁺ decreasing to ~1 μ_B in low-spin (LS) state. Meanwhile, the reductions of cell volume (by ~?5.43%) and band gap (from >2 eV to ~1.6 eV) of SmFeO₃ are obtained when the HS–LS transition happens. Finally, the critical pressure of HS–LS transition, magnetic and electronic properties are found to be Hubbard U dependent.     

TiN多型体高压相变的第一性原理计算

基于密度泛函理论框架下的赝势平面波方法,计算了B1(氯化钠结构)、B2(氯化铯结构)、B3(闪锌矿结构)、B_k(六方氮化硼结构)、B_h(碳化钨结构)和B8₁(砷化镍结构)6种TiN多型体的晶体结构、体积弹性模量和相对稳定性.计算指出,不存在B4(纤锌矿)结构的TiN.通过不同外压下的晶格弛豫得到每种结构的焓,发现外压 关键词： 氮化钛 赝势 高压相变 密度泛函理论     

Structural and thermodynamic properties of OsN2 from first-principles calculations

We investigate the structural and thermodynamic properties of OsN2 by a plane-wave pseudopotential density functional theory method. The obtained lattice constant,bulk modulus and cell volume per unit formula are consistent with the available theoretical data. Moreover,the pressure-induced phase transition of OsN2 from pyrite structure to fluorite structure has been obtained. It is found that the transition pressure of OsN2 at zero temperature is 67.2 GPa. The bulk modulus B as well as other thermodynamic quantities of fluorite OsN2 (including the Gru¨neisen constant γ and thermal expansion α) on temperatures and pressures have also been obtained.     

Pressure-induced structural and electronic transition in KTb(MoO4)2 through Raman and optical studies

Raman and optical absorption studies under pressure have been conducted on KTb(MoO₄)₂ up to 35.5 GPa. A phase transformation occurs at 2.7 GPa when the crystal is pressurized at ambient temperature in a hydrostatic pressure medium. The sample changes to a deep yellow color at the transition and visibly contracts in theα-axis direction. The color shifts to red on further pressure increase. The Raman spectral features and the X-ray powder pattern change abruptly at the transition indicating a structural change. The pressure-induced transition appears to be a property of the layer-type alkali rare earth dimolybdates. However, the color change at the transition in KTb(MoO₄)₂ is rather unusual and is attributed to a valence change in Tb initiated by the structural transition and consequent intervalence charge transfer between Tb and Mo.In situ high pressure X-ray diffraction data suggest that phase II could be orthorhombic with a unit cell having 3 to 4% smaller volume than that of phase I.     

Pressure-induced structural phase transition and electronic properties of RESb (RE = Ho,Er and Tm) compounds: ab initio calculations

The structural phase transition and electronic properties at ambient (B ₁-phase) and high pressure (B ₂-phase) of heavy rare earth monoantimonides (RESb; RE?=?Ho, Er, and Tm) have been studied theoretically using the self-consistent tight binding linear muffin tin orbital method. These compounds show metallic behavior under ambient condition and undergo a structural phase transition to the B ₂ phase at high pressure. We predict a structural phase transition from the B ₁ to B ₂ phase in the pressure range 30.0–35.0?GPa. Apart from this, the ground state properties, such as lattice parameter and bulk modulus are calculated and compared with the available theoretical and experimental results.     

Raman scattering study of pressure-induced phase transitions in AIIB2IIIC4VI defect chalcopyrites and spinels

A^IIB₂^IIIC₄^VI defect chalcopyrites (DC) and spinels were investigated by Raman scattering spectroscopy under hydrostatic pressure up to 20 GPa. All these compounds were found to undergo a phase transition to a Raman inactive defect NaCl-type structure. The phase transition is reversible for spinels and irreversible for DC. From the analysis of the pressure behavior of Raman-active modes, it was concluded that the phase transition from spinel to NaCl-type structure is direct in MnIn₂S₄ and CdIn₂S₄, while it occurs via an intermediate LiVO₂-type NaCl superstructure in MgIn₂S₄. The observed differences in the pressures and the paths of the pressure-induced phase transitions in A^IIB₂^IIIC₄^VI compounds are discussed.     

The high-pressure structural transition in MnS: an ab initio study

The structural phase transition between B1 (α-MnS) and B3 (β-MnS) is investigated using a density functional theory method. The structural phase transition pressure Pt from α-MnS to β-MnS, which is determined on the basis of the third-order Birch–Murnaghan equation of states, is 30.75?GPa. Also, the lattice parameters a, the bulk modulus B and pressure derivative of bulk modulus B′, which are generally in good agreement with experiments and other theoretical values, are obtained under zero pressure. For further investigation of the structural phase transition pressure of MnS, the relative volumes V/V ₀, the bulk modulus B, first and second pressure derivatives (B′ and B″) of bulk modulus for the two structures of MnS have been calculated under various pressures.     

AlAs相变及热动力学性质的第一性原理研究

利用基于密度泛函理论的全势能线性糕模轨函法研究了闪锌矿（B3）,NiAs（B8）和岩盐（B1）结构的AlAs的相变、结构性质以及热动力学性质.对B3-B8和B3-B1结构的能量体积曲线做公切线,得到了B3→B8相变压力为5.44 GPa,并预测到B3→B1相变压力为6.46 GPa.同时计算了高压下B8相的结构性质,结果显示当V/V₀≈0.7—1.05时,c/a基本保持恒定（仅有约 0.2%的波动）;当V/V₀≈0.4—0.7,c/a随着V/V₀的减小而近似线性地增大.通过状态方程拟合,得到了AlAs的相对体积V/V₀与压强P的函数关系,B8相的状态方程与实验结果符合很好.最后利用准谐德拜模型得到了AlAs的体弹模量B随压力P的变化关系以及不同压强下热容C_V与温度T的关系. 关键词： 相变 热力学性质 第一性原理     

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.     

Phase transitions of YbX (X = P,As and Sb) with a NaCl-type structure at high pressures

The powder X-ray diffraction of YbX (X?=?P, As and Sb) with a NaCl-type structure has been studied with synchrotron radiation up to 63?GPa at room temperature. YbSb undergoes the first-order structural phase transition from the NaCl-type (B1) to the CsCl-type (B2) structure at around 13?GPa. The structural change to the B2 structure occurs with the volume collapse of about 1% at 13?GPa. The transition pressure of YbSb is surprisingly lower than that of any other heavier LnSb (Ln?=?Dy, Ho, Er, Tm and Lu). The pressure-induced phase transitions in YbP and YbAs are observed at around 51?GPa and 52?GPa respectively. The transition pressure of both compounds is much higher than that of YbSb. The high-pressure structural behaviour of YbX (X?=?P, As and Sb) is discussed. The volume versus pressure curve for YbX with the NaCl-type structure is fitted by a Birch equation of state. The bulk moduli of these compounds with the NaCl-type structure are 104?GPa for YbP, 85?GPa for YbAs and 52?GPa for YbSb.     

Phase transformation and elastic behavior of MgX (X=S, Se, Te) alkaline earth chalcogenides

Pressure induced structural aspects of NaCl-type (B1) to CsCl-type (B2) structure in alkaline earth chalcogenides (AECs) magnesium chalcogenides (MgX; X=S, Se, and Te) are presented. An effective interionic interaction potential (EIoIP) with long-range Coulomb interactions 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 developed. The vdW coefficients are evaluated following the Slater-Kirkwood variational method, as both the ions are polarizable. The present calculations have revealed reasonably good agreement with the available experimental data on structural transition (B1-B2 structure), the phase transition pressures P_t of 167 (MgS), 170 (MgSe), and 176 (MgTe) GPa as well the elastic properties. The calculated values of the volume collapses [ΔV(P)/V(0)] are also closer to their observed data. 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 semiconducting compounds with rocksalt (B1) type crystal structure. The Born and relative stability criteria is valid in Mg monochalcogenides.     

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.     

Investigations of phase transition, elastic and thermodynamic properties of GaP by using the density functional theory

The phase transition of gallium phosphide (GaP) from zinc-blende (ZB) to a rocksalt (RS) structure is investigated by the plane-wave pseudopotential density functional theory (DFT). Lattice constant a₀, elastic constants c_ij, bulk modulus B₀ and the pressure derivative of bulk modulus B₀' are calculated. The results are in good agreement with numerous experimental and theoretical data. From the usual condition of equal enthalpies, the phase transition from the ZB to the RS structure occurs at 21.9 GPa, which is close to the experimental value of 22.0 GPa. The elastic properties of GaP with the ZB structure in a pressure range from 0 GPa to 21.9 GPa and those of the RS structure in a pressure range of pressures from 21.9 GPa to 40 GPa are obtained. According to the quasi-harmonic Debye model, in which the phononic effects are considered, the normalized volume V/V₀, the Debye temperature θ, the heat capacity C_v and the thermal expansion coefficient α are also discussed in a pressure range from 0 GPa to 40 GPa and a temperature range from 0 K to 1500 K.