Ab initio molecular dynamics simulation of pressure-induced phase transformation in BeO

Ab initio molecular dynamics (MD) method has been used to study high pressure-induced phase transformation in BeO based on the local density approximation (LDA) and the generalized gradient approximation (GGA). Both methods show that the wurtzite (WZ) and zinc blende (ZB) BeO transforms to the rocksalt (RS) structure smoothly at high pressure. The transition pressures obtained from the LDA method are about 40 GPa larger than the GGA result for both WZ → RS and ZB → RS phase transformations, and the phase transformation mechanisms revealed by the LDA and GGA methods are different. For WZ → RS phase transformations both mechanisms obtained from the LDA and GGA methods are not comparable to the previous ab initio MD simulations of WZ BeO at 700 GPa based on the GGA method. It is suggested that the phase transformation mechanisms of BeO revealed by the ab initio MD simulations are affected remarkably by the exchange–correlation functional employed and the way of applying pressure.     

A density functional study of structural and elastic properties of LaN under high pressure

Structural and elastic properties of LaN at normal and high pressures are investigated using ab initio calculations based on full-potential linearized augmented plane wave (FP-LAPW) within both local density approximation (LDA) and generalized gradient approximation (GGA). Our results concerning equilibrium lattice parameter and bulk modulus agree well with the available experimental and previous theoretical findings. The transition pressure from NaCl (B1) to CsCl (B2) phase is found to be 31.05 GPa from LDA, and 42.2 GPa from GGA. To the best of our knowledge, the elastic properties for LaN in the B1 structure in the presence of pressure have never been reported so far. The linear pressure coefficients of elastic constants and their related bulk modulus are determined from the pressure dependence of these parameters. Furthermore, the mechanical stability criteria for LaN in B1 phase are found to be fulfilled at normal conditions.     

Structural phase stability of ThSb and ThAs under pressure

The high-pressure behaviour of thorium monopnictides is of considerable interest as these systems exhibit structural phase transitions under pressure. At ambient conditions these compounds crystallize in the NaCl-type (B1) structure. Experiments show that with the application of pressure these compounds transform to the CsCl-type (B2) structure. ThSb and ThAs are found to exhibit B1–B2 transition in the pressure range between 9–12 GPa and 1826 GPa respectively. In this work, we present the electronic and high-pressure behaviour of ThAs and ThSb performed using the tight-binding linear muffin-tin orbital method. The total energies within the atomic sphere approximation were calculated as a function of volume for both the B1 and B2 structures. The total energy calculations reveal that both ThSb and ThAs are stable in the B1 structure at ambient conditions and undergo structural transition to the B2 structure at pressures 78 and 240 kbar respectively, which are in good agreement with the experimental values. The calculated values of equilibrium lattice parameter and the transition pressure are found to be in good agreement with the experimental results.     

First-principles study of structural and elastic properties of MgSe under hydrostatic pressure

The structural and elastic properties were calculated using ab initio plane wave pseudopotential method within the generalized gradient approximation (GGA). Our results indicated that MgSe undergoes a structural phase transition from NaCl-type (B1) to FeSi-type (B28) at a pressure near to about 111 GPa. The calculated elastic stiffness coefficients presented a linear behaviour versus pressure. The structural parameters and elastic constants of the fundamental ground are generally in good agreement with the available theoretical and experimental data.     

Effect of pressure on the structural and elastic properties of ZnS and MgS alloys in the B3 and B1 phases

From the first-principles calculations, we have investigated the elastic stiffness coefficients C₁₁, C₁₂, C₄₄ and the bulk modulus B of the II-VI semiconductors ZnS and MgS under hydrostatic pressure. The calculations are based on the density functional theory within the generalized gradient approximation (GGA) for exchange-correlation interaction. For the structural properties we have shown that ZnS adopt the rocksalt (NaCl or B1) structure over 11.87 GPa pressure, the same character is adopted by MgS over 0.8 GPa. The elastic coefficients have the same behavior for the different structures of alloys; they increase with increasing pressure values. Our results for the structural parameters and equilibrium phase elastic constants are in good agreement with the available theoretical and experimental data.     

High-pressure structural phase transitions and mechanical properties of calcite rock

In the present work, a state of the art first principles theory is used to examine the structural and mechanical properties of calcium carbonates CaCO₃. Our calculations allowed full structural relaxation, which permits an appropriate evaluation of material properties at ambient conditions as well as under hydrostatic pressure. Compared to experimental measurements the calculated ground state properties show a suitable agreement. By performing a structural phase stability analysis, we were able to predict both first and second order phase transitions that calcium carbonates minerals undertake under hydrostatic pressure. The first one occurs between the calcite and aragonite phases at 3.3 GPa and the second one between the aragonite and post-aragonite phases at ～40 GPa. The previous value agree very well with experimental one (40 GPa) reported by Ono et al. In order to verify the reliability of such phase transitions, we study the mineral high pressure stability by means of mechanical properties behaviour. Both transversal wave velocity and elastic moduli show an unexpected decrease at phase transition pressure range.     

Pressure-induced phase transition of tantalum mononitride

Three synthesized stable phases for TaN are studied by the first-principles calculations. We confirm that TaN undergoes a phase transition from the hexagonal ? phase to the hexagonal θ one with a volume collapse of 5.25% when the applied pressure is 3.2 GPa. The obtained phase transition pressure is in good agreement with the experimental results. An analysis of the calculated results shows that this phase transition is mainly caused by the higher bulk modulus and lower electron states on the Fermi level of the hexagonal θ phase for TaN. The superior elastic properties indicate that TaN in the hexagonal θ phase is an ultra-incompressible and hard material.     

B3–B1 phase transition and pressure dependence of elastic properties of ZnS

We have performed the ab initio calculations based on density functional theory to investigate the B3–B1 phase transition and mechanical properties of ZnS. The elastic stiffness coefficients, C₁₁, C₁₂, C₄₄, bulk modulus, Kleinman parameter, Shear modulus, Reuss modulus, Voigt modulus and anisotropy factor are calculated for two polymorphs of ZnS: zincblende (B3) and rocksalt (B1). Our results for the structural parameters and elastic constants at equilibrium phase are in good agreement with the available theoretical and experimental values. Using the enthalpy–pressure data, we have observed the B3 to B1 structural phase transition at 18.5 GPa pressure. In addition to the elastic coefficients under normal conditions, we investigate the pressure dependence of mechanical properties of both phases: up to 65 GPa for B1-phase and 20 GPa for B3-phase.     

Structural phase transition,electronic and elastic properties in TlX (X = N,P, As) compounds: Pressure-induced effects

Using the first-principles plane-wave pesudopotential (PW-PP) method with the generalized gradient approximation (GGA) for the exchange-correlation potential, we have studied the structural, electronic and elastic properties of TlX (X = N, P, As) compounds under hydrostatic pressure. Our calculations show that TlN, TlP and TlAs undergo a phase transition from the zincblende (ZB) to the rocksalt (RS) structure at 19.05, 7.29 and 5.01 GPa, with a volume collapse of 14.96%, 17.45% and 18.03%, respectively. The influence of crystallographic structure and hydrostatic pressure on electronic band structures, elastic constants, and aggregate bulk moduli are investigated. Further, the elastic wave velocities, Debye temperatures and melting temperatures for zincblende TlN, TlP and TlAs compounds are obtained. Our calculated results are compared with the previously reported theoretical data.     

Structural,electronic, elastic properties and stabilities of hexagonal ZrNiAl alloy and its hydride ZrNiAlH0.67 under pressure

The structural, electronic, elastic properties and stabilities of hexagonal prototype alloy ZrNiAl and its saturated hydride ZrNiAlH_0.67 are investigated using the pseudopotential plane wave method within the generalized gradient approximation (GGA). The calculated structural parameters are in good agreement with the available experimental data. Partial covalent characters on ZrNiAl and ZrNiAlH_0.67 are verified by the calculations of PDOS (partial density of states) and overlap population. Band structures show both ZrNiAl and ZrNiAlH_0.67 belong to metals. The elastic constants and their pressure dependences are calculated using the static finite strain technique. From the analysis of the mechanical stabilities, hexagonal ZrNiAl is unstable at higher pressure than 29.34 GPa; that its hydride ZrNiAlH_0.67 is stable up to 50 GPa is similar with the experimental result of isostructural LaNiInD_1.63−x. Hydrogenation not only leads to strong lattice anisotropy but also leads to strong mechanical anisotropy.     

Theoretical study of structural,electronic, and thermal properties of CdS,CdSe and CdTe compounds

A theoretical study of structural, electronic and thermal properties of CdS, CdSe and CdTe compounds is presented; using the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). In this approach, both the local density approximation (LDA) and the generalized gradient approximation (GGA) were used for the exchange-correlation potential calculation. The ground-state properties are determined for the bulk materials (CdS, CdSe and CdTe) in cubic phase. Quantities such as the lattice constants and bulk modulus of interest are calculated. Detailed comparisons are made with published experimental and theoretical data and show generally good agreement. Besides this, a numerical first-principles calculation of the elastic constants was used to calculate C₁₁, C₁₂ and C₄₄. The pressure dependence of band gaps for these systems was investigated. We also presented the thermal effects on some macroscopic properties of these compounds using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. We have obtained successfully the variations of the volume, thermal expansion coefficient, heat capacities and Debye temperature as a function of the pressure and temperature.     

First-principles study of zinc-blende to rocksalt phase transition in BP and BAs

The structural and electronic properties of BP and BAs are investigated by first-principles pseudopotential method. The calculations show the structural phase transition from zinc-blende (ZB) structure to rocksalt (RS) structure at the transition pressure of 142 GPa for BP and 134 GPa for BAs. The ZB phase of BP and BAs remains indirect gaps upon applying hydrostatic pressure, while RS phase of BP and BAs is semimetal at the transition pressure.     

First-principles study of the electronic,optical and bonding properties in dolomite

The structural, electronic, optical properties and chemical bonding of dolomite CaMg(CO₃)₂ (rhombohedral calcite-type structure) are investigated using plane wave pseudopotential density-functional theory (DFT) method taking the local density approximation (LDA) and the generalized gradient approximation (GGA) as the exchange–correlation energy functional. The structural properties are consistent with the early experimental and theoretical results. The indirect electronic band gap is estimated to be ～5.0 eV, which is less than the optical band gap measured from the fundamental absorption edge of ～6.0 eV. The optical band gap is also consistent with the experimental band gap of similar calcite-type structure. A noticeable difference for the LDA and GGA derived transition peaks and a significant optical anisotropy are observed in the optical spectra. The analysis of electronic density of states, Mulliken charge and bonding population shows the coexistence of covalent and ionic bonding in the dolomite structure and the results are consistent with previous theoretical calculations.     

FP-LAPW study of the fundamental properties of the cubic spinel CdAl2O4

We have investigated the structural, elastic, electronic, optical and thermodynamic properties of the cubic spinel CdAl₂O₄ using accurate ab initio calculations. Computed equilibrium structural parameters are in good agreement with the available experimental data. Single-crystals elastic parameters are calculated for pressure up to 30 GPa using a conserving-volume total energy-strain method. Isotropic elastic parameters for ideal polycrystalline CdAl₂O₄ aggregates are computed in the framework of the Voigt-Reuss-Hill approximation. Result for band structure using the Engel-Vosko scheme of the GGA shows a significant improvement over the common GGA functionals. Optical spectra have been calculated for the energy range 0-30 eV. The peaks and structures in the optical spectra are assigned to interband transitions. Pressure dependence of the band gaps, static dielectric constant and static refractive index are also investigated. Pressure and thermal effects on some macroscopic properties are predicted using the quasi-harmonic Debye model.     

First-principles study of phase transition and structural properties of AlAs

We have investigated the phase transition and structural properties of AlAs in three crystallographic structures, i.e., B3 (zinc blende), B1 (rocksalt), and B8 (nickel arsenide), at high pressures using the full-potential linearized muffin-tin orbital (FP-LMTO) scheme within the generalized gradient approximation correction (GGA) in the frame of density functional theory (DFT). For B8 structure, it is found that the c/a ratios kept nearly constant (0.2% fluctuation) corresponding to V/V₀  0.7–1.05 (V is the primitive cell volume and V₀ is the experimental equilibrium volume of B3 structure), which is in full agreement with experiment, but the c/a ratios increase linearly with the values of V/V₀ decreasing corresponding to V/V₀  0.4–0.7. This indicates under low pressure the compression along c-axis and a-axis is the same, but the compression along c-axis is more difficult than along a-axis under higher pressure. Based on the condition of equal enthalpies AlAs is found to undergo a structural phase transition from B3 to B8 at 5.34 GPa, in agreement with the experimental value of 7 ± 5 GPa, and is speculated to undergo the B3–B1 transition at 6.24 GPa.     

Structural,thermodynamic, electronic,and optical properties of NaH from first-principles calculations

The structural stability, thermodynamic, electronic, and optical properties of NaH with rock salt (B1) structure and cesium chloride (B2) structure under high pressure are investigated by first-principles calculations using norm-conserving pseudopotential applying a generalized gradient approximation (GGA) for exchange and correlation. Through the analysis of energy–volume variation, we find the phase transition of NaH from B1 to B2 structure occurs at 32.3 GPa, which in good agreement with the diamond-anvil-cell high-pressure experimental value of 29.3 ± 0.9 GPa [Phys. Rev. B 36 (1987) 7664]. By using the quasi-harmonic Debye model, the thermodynamic properties including the Debye temperature Θ_D, heat capacity C_V, thermal expansion coefficient α, and Grüneisen parameter γ are successfully obtained in the temperature range from 0 to 700 K and pressure ranges from 0 to 32 GPa and 33 to 100 GPa for NaH B1 and B2 phases, respectively. Analysis of band structures suggests that the NaH has an indirect band gap that the valence band maximum is at the W point and the conduction minimum locates at L point. The calculated energy gaps is very close to that value obtained in recent full potential augmented plane wave calculations. The optical properties including dielectric function ?(ω), absorption coefficient α(ω), reflectivity coefficient R(ω), and refractive index n(ω) are also calculated and analyzed.     

Ab initio prediction of elastic and thermal properties of cubic TiO2

In this paper we focus on the novel solar material, namely cubic TiO₂. The full potential linearized augmented plane wave method in combination with the local density approximation (LDA) and the generalized gradient approximation (GGA) have been used. We calculated structural parameters, elastic constants, wave velocities and thermal properties of the material assuming the fluorite structure. The obtained values were in good agreement with the available theoretical and experimental data. Moreover, the pressure and temperature dependences of the bulk modulus, Debye temperature, Heat capacity and linear expansion coefficient have been addressed for the first time.     

Effect of pressure on structural and elastic properties of alkaline-earth oxide CaO

We have performed ab initio calculations using a plane wave pseudopotential method to investigate the phase transition of alkaline-earth oxide CaO from NaCl (B1) to CsCl (B2) type structure. The elastic constants for this material have been determined in the pressure range 0–140 GPa. Also, the effect of hydrostatic pressure on the propagation elastic waves has been studied. The specific elastic constants, bulk modulus and wave velocities that we calculated for both B1 and B2 type structures are in good agreement with the available experiment data.     

Ab-initio studies of pressure induced phase transitions in BaO

We use ab-initio Quantum Mechanics to study the zero temperature phase diagram of BaO. We calculate zero temperature Equations of State of different crystalline phases [B1 (NaCl), B8(NiAs), B2(CsCl), and distorted B2] using Density Functional Theory (DFT) with the generalized gradient approximation (GGA). We find the B1 structure to be the thermodynamically stable one at zero pressure; followed by three pressure induced phase transitions. We find that at P=11.3 GPa BaO transforms from B1 to B8; at P=21.5 GPa from B8 to distorted B2. The distorted B2 phase continuously approaches the B2 structure, the phase transformation occurs at P=62 GPa. We also study the band structure of BaO in its high pressure (B2) phase. For P=60.5 GPa, we find a band gap of 3.5 eV in agreement with experimetal value. We find metallization at P=230.6 GPa.