Pressure-induced phase transition and structural behavior of MgO at high temperatures: a model study

A realistic interaction potential model approach by including temperature effects is developed to study phase transition, elastic properties and thermo-physical properties at very high pressures and temperatures. This approach is effectively able to explain the inter-atomic interaction involved at high temperature and high pressure as it includes the three-body interactions. Earlier works overlooked the three-body interactions at high temperature and pressures. Moreover, the phase-transition pressures of MgO crystal at high temperatures including the three-body interaction are computed for the first time. Elastic behavior, anisotropic factor and Debye temperature of MgO at high pressures and temperatures are also reported.     

Analysis of structural properties of refractory compounds at high temperature and pressure using a potential model

In this paper we focused on the structural and elastic properties of four transition metal mononitrides (TMNs) (M=Ti, Nb, Hf and Zr) by using realistic three body interaction potential (RTBIP) model, including the role of temperature. These TMN compounds have been found to undergo NaCl (B1) to CsCl (B2) phase transition, at a pressure quite high as compared to other binary systems. We successfully obtained the phase transition pressures and volume changes at different temperatures. In addition, elastic constants of TMNs at different temperatures are discussed. The present theoretical results have been compared with the available experimental data and predictions of LDA theory.     

Phase transition,structural, mechanical and thermal properties of erbium pnictides under pressure

In this article, we have investigated the high-pressure structural phase transition of erbium pnictides (ErX; X?=?N, P and As). An extended interaction potential model has been developed (including the zero-point energy effect in three-body interaction potential model). Phase transition pressures are associated with a sudden collapse in volume. The phase transition pressures and associated volume collapses have been predicted successfully. The elastic constants, their combinations and pressure derivatives are also reported. The pressure behaviour of elastic constants, bulk modulus and shear modulus have been presented and discussed. Moreover, the thermophysical properties such as molecular force constant (f), infrared absorption frequency (υ ₀), Debye temperature (θ _D) and Grunneisen parameter (γ) have also been predicted.     

Temperature and pressure behaviour of narrow-gap semiconductors including galena

The structural high pressure and temperature investigation of narrow-gap semiconductors (lead chalcogenides) has been performed in the present article. A realistic approach for room temperature and high temperature study of narrow-gap semiconductors has been used. It is examined that the present compounds are more stable in NaCl-phase and they transform to CsCl-phase at high pressure. In the present article, the phase transition pressures and volume collapses of lead chalcogenides have been investigated at room and high temperatures. Phase transition pressures have been reported at high temperature range from 0 to 1200 K. Elastic and anharmonic constants have also been reported at room temperature. A structural study of the narrow-gap semiconductors have been carried out using the realistic model including temperature effect. The temperature and pressure behaviour of elastic constants for the present compounds have also been discussed. Furthermore, various mechanical and thermo dynamical properties like modulus of elasticity, Debye temperatures etc. are also presented.     

密度泛函理论研究高温高压下UO2弹性与热力学性能

采用第一性原理与准谐德拜模型研究UO2在高温高压条件下的弹性与热力学性能。UO2在高温高压下仍属离子型晶体,并且弹性性能计算表明,四角方向剪切常数在高温与高压下均保持稳定。高温下弹性常数C44没有明显变化,而高压下C44迅速增大。体积模量、剪切模量与杨氏模量均随压强增加而增大;高温条件下,体积模量、剪切模量与杨氏模量也未出现明显的降低,表明UO2在高温度高压下均可保持良好的力学性能。不同压强下,UO2定容热容均随温度迅速增大,并在1000 K 附近趋近于杜隆-佩蒂特极限。德拜温度则随温度降低,随压强升高。在低于室温条件下,热膨胀系数随温度急剧增加;温度继续增加,系数的增加趋势则逐渐变缓。计算结果还表明,UO2的热膨胀系数在相同条件下,远小于其他核材料。     

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.     

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.      

Thermoelectric behaviour of SmS and Sm0.84Gd0.16S at high pressures and temperatures

The 4f→ 5d electronic phase transition in SmS has been studied using thermoelectric power as a probe. The variation of thermopower with pressure in Sm_0.84Gd_0.16S and in the high pressure phase of SmS is anomalous, characterized by a rather large pressure coefficient. The temperature coefficient of thermopower in Sm_0.84Gd_0.16S is large and negative at low pressures leading to a change of sign at higher temperatures. The anomalies can be understood on the basis of the ICF model.     

Neutron scattering study of the phase transition in s-triazine at high pressure

The elastic transition in s-triazine (C₃N₃H₃) from a trigonal ($\text{R}\text{3}\text{c}$) high temperature (low pressure) structure to a monoclinic (C2/c) low temperature (high pressure) phase has been investigated at pressures up to 5 kbar using neutron scattering techniques. Neutron diffraction was used to measure the pressure dependence of the order parameter and inelastic scattering to study the softening of the transverse acoustic phonon modes on three isotherms. In both cases the effect of pressure on the transition is found to be described primarily by that on the temperature of the transition.     

First-principles study of the high pressure phase transition and lattice dynamics of cerium

We present a first-principles study of the phase transition and lattice dynamics of Ce within the framework of the density functional theory using the GGA+U method. Our calculated results denote that under pressure the transition path is α-Ce (fcc)→α″-Ce (monoclinic, with two atoms per unit cell)→bct-Ce (body centered tetragonal), and the transition pressures are located at 5.36 and 14.37 GPa, respectively. The equation of state in a wide range of pressure is consistent with the experimental data. During the γ-α phase transition, the magnetic moment disappears gradually, which is mainly due to the strong interaction between the 4f and 5d electrons. By calculating the free energies from phonon dispersions including electronic contribution, the obtained γ-α transition temperature at zero pressure is 148 K. From the Blackman diagram of dimensionless elastic constant ratios, we can find that both γ- and α-Ce show negative Cauchy pressure—C₄₄>C₁₂.     

Pressure-induced phase transition and stability of EuO and EuS with NaCl structure

We have predicted the phase transition pressures and corresponding relative volume changes of EuO and EuS having NaCl-type structure under high pressure using three-body interaction potential (TBIP) approach. In addition, the conditions for relative stability in terms of modified Born criterion has been checked. Our calculated results of phase transitions, volume collapses and elastic behaviour of these compounds 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.      

高压下TiC的弹性、电子结构及热力学性质的第一性原理计算

利用基于密度泛函理论的第一性原理平面波赝势方法 并结合准谐徳拜模型研究了NaCl结构的TiC在高压下的弹性性质、电子结构和热力学性质. 计算所得零温零压下的晶格常数、体弹模量及弹性常数与实验值符合得很好. 零温下弹性常数和弹性模量随压强增大而增大. 通过态密度和电荷密度的分析, Ti-C键随压强增大而增强. 运用准谐德拜模型, 成功计算了TiC在高温高压下的体弹模量、熵、热膨胀系数、徳拜温度、 Grüneisen参数和比热容. 结果表明压强对体弹模量、热膨胀系数和徳拜温度的影响大于温度对其的影响. 热容随着压强升高而减小, 在高温高压下, 热容接近Dulong-Petit极限.     

Pressure and temperature induced high spin–low spin phase transition: Macroscopic and microscopic consideration

The behavior under pressure of the high spin–low spin phase transition in the coordination compounds containing 3d ions is analyzed using thermodynamic and microscopic approaches. For thermodynamic approach the mean field model with interactions between spin-crossover molecules is considered. Microscopic model takes into account the interaction of d electrons of the transition metal ions with full symmetric distortions of the ligands. The relationship of the thermodynamic interaction parameters with microscopic ones is installed and shown how the quantum–mechanical interactions form the cooperativity of the system. Within the microscopic model the temperature and pressure dependences of the high spin fraction in 2-D compounds {Fe(3-Fpy)₂[M(CN)₄]} (M=Pd, Pt) are simulated and microscopic parameters are evaluated. It is concluded that different experimental behaviors of the temperature and pressure induced spin transitions are determined by different variations of the inelastic and elastic energies under pressure, and vibrational component of the free energy drives the ST equally with electronic part.     

Structural stability of TiO and TiN under high pressure

The high pressure phase transition and elastic behavior of Transition Metal Compounds (TiO and TiN) which crystallize in NaCl-structure have been investigated using the three body potential model (TBPM) approach. These interactions arise due to the electron-shell deformation of the overlapping ions in crystals. The TBP model consists of a long range Coulomb, three body interactions, and the short-range overlap repulsive forces operative up to the second neighboring ions. The authors of this paper estimated the values of the phase transition pressures, associated volume collapses, and elastic constants, all of which were found to be closer to available experimental data than other calculations. Thus, the TBPM approach promises to predict the phase transition pressure and pressure variations of elastic constants of Transition Metal compounds.      

Structural study of ND4I at high pressures and low temperatures

Abstract

Structure, positional, and thermal parameters of ND₄I were studied at high pressures up to 90 kbar and low temperatures down to 10 K using time-of-flight neutron diffraction. The phase transition from a disordered CsCI-type cubic phase ND₄I(II) into a recently discovered high pressure phase ND₄I(V) was observed at P = 80(5) kbar. Surprisingly, the structure of the high pressure phase V was found to bear a strong resemblance to that of the ambient pressure, low-temperature phase III - tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm. The critical value of the deuterium positional parameter corresponding to the II-V transition is close to the one for the phase transition between the disordered and ordered CsCl-type cubic phases II and IV in other ammonium halides.     

Elastic properties of alkaline earth oxides under high pressure

In this article, we have investigated the high-pressure structural phase transition of alkaline earth oxides using the three-body potential (TBP) model. Phase transition pressures are associated with elastic constants. An effective inter-ionic interaction potential (TBP) with long-range Coulomb interactions and the Hafemeister–Flygare type short-range overlap repulsion and the vdWl interaction is developed. The present calculations have revealed reasonably good agreement with the available experimental data on structural transition (B1–B2 structure). The phase transition pressures Pt of MgO, CaO, SrO, and BaO occur at 220, 45, 40, and 100?GPa, respectively. Further, the variations of the second-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. It is found that TBP promises that we would be able to predict phase transition pressure and elastic constants for other chalcogenides as well. The results may be useful for geophysical study.     

高温高压下闪锌矿相GaN结构和热力学特性的分子动力学研究

利用分子动力学方法和Buckingham经验势模型对重要半导体材料GaN立方闪锌矿相的晶格常数、相变压力(从闪锌矿到岩盐结构)、热膨胀、等温体模量、定压热容等结构和热力学特性在300—3000K的温度范围和0—65GPa的压力范围内进行了研究.研究表明，闪锌矿相GaN常态下的结构和热力学参数的模拟结果与实验数据及其他理论结果相符.同时在所选作用势模型可靠性检验的基础上，对等温体模量、定压热容诸非谐性参量在高温高压下的热力学行为进行了预测.所得结果在材料科学等领域的研究中具有一定的应用背景和参考价值. 关键词： GaN Buckingham势 分子动力学模拟 高温高压     

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.      

Neutron diffraction and electrical transport studies on the incommensurate magnetic phase transition in holmium at high pressures

Neutron diffraction and electrical transport measurements have been made on the heavy rare earth metal holmium at high pressures and low temperatures in order to elucidate its transition from a paramagnetic (PM) to a helical antiferromagnetic (AFM) ordered phase as a function of pressure. The electrical resistance measurements show a change in the resistance slope as the temperature is lowered through the antiferromagnetic Néel temperature. The temperature of this antiferromagnetic transition decreases from approximately 122 K at ambient pressure at a rate of -4.9 K GPa(-1) up to a pressure of 9 GPa, whereupon the PM-to-AFM transition vanishes for higher pressures. Neutron diffraction measurements as a function of pressure at 89 and 110 K confirm the incommensurate nature of the phase transition associated with the antiferromagnetic ordering of the magnetic moments in a helical arrangement and that the ordering occurs at similar pressures as determined from the resistance results for these temperatures.     

A first principles lattice dynamics and Raman spectra of the ferroelastic rutile to CaCl2 phase transition in SnO2 at high pressure

A first principles calculation of the lattice dynamical properties of rutile SnO₂ has been performed using density functional perturbation theory at ambient and high‐pressure conditions to understand the pressure‐induced phase transition. The calculated zone centre phonon modes at ambient and high pressures have been compared with Raman scattering and infrared measurements. Full phonon dispersion curves and phonon densities of states and Raman intensities at high pressures are calculated and given for the first time in literature. The ferroelastic transition from the rutile to the CaCl₂‐type structure was confirmed. It is clearly illustrated that the first transition is associated with macroscopic shear instability which arises from the strong coupling between elastic constants and softening of Raman active B_1g mode. The observed pressure of phase transition in experimental measurements was reproduced more accurately than in previous calculations, and the difference between observed and calculated transition pressure is only of the order of 2%. The mode Grüneisen parameter is quantitatively as well as qualitatively different from the earlier reported values. Copyright © 2013 John Wiley & Sons, Ltd.