First-principles study of structural stability, electronic and elastic properties of ZrC compounds

We theoretically study the possible pressure-induced structural phase transition, electronic and elastic properties of ZrC by using first-principles calculations based on density functional theory (DFT), in the presence and absence of spin-orbit coupling (SOC). The calculations indicate that there exists a phase transition from the NaCl-type (B1) structure to CsCl-type (B2) structure at the transition pressure of 313.2 GPa (without SOC) and 303.5 GPa (with SOC). The detailed structural changes during the phase transition were analyzed. The band structure shows that B1-ZrC is metallic. A pseudogap appears around the Fermi level of the total density of states (DOS) of the B1 phase of ZrC, which may contribute to its structural stability.     

Theoretical study of structural energy, phonon spectra, and elastic constants of Rh and Ir

The transition metal pair potential (TMPP) is used to study band structure energy of Rh and Ir. Both metals are found to be most stable in fcc structure down to atomic volume 0.5V ₀. The pressure at 0.5V ₀ is found to be 5.235 Mbar and 9.216 Mbar in Rh and Ir, respectively. The TMPP is also used to study other properties of these metals like cohesive energy, phonon frequencies at observed volume. The bulk moduli and elastic constants of these metals at observed volume are calculated by including the volume contribution.     

Theoretical investigation of the elastic,electronic, thermodynamic and optical properties of the zinc-blende structure AlN under high pressure

The elastic, electronic, thermodynamic and optical properties of the zinc-blende structure aluminum nitride (AlN) under high pressure have been investigated using first-principles calculations. The dependencies of the elastic constants, the bulk modulus, the shear modulus and energy gaps on the applied pressure are presented, and the results are in good agreement with comparable experimental and theoretical values. Also, the energy band structure and density of states under high pressure have been analysed. Furthermore, the optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are discussed for radiation up to 50 eV.     

First-principles investigation on the structural and elastic properties of cubic-Fe,TiAl under high pressures

The structural, elastic, and thermodynamic properties of cubic-Fe2TiA1 under high temperatures and pressures are investigated by performing ab initio calculation and using the quasi-harmonic Debye model. Some ground state properties such as lattice constant, bulk modulus, pressure derivative of the bulk modulus, and elastic constants are in good agreement with the available experimental results and theoretical data. The thermodynamic properties of Fe2TiA1 such as thermal expansion coefficient, Debye temperature, and heat capacity in ranges of 0 K-1200 K and 0 GPa-250 GPa are also obtained. The calculation results indicate that the heat capacities at different pressures all increase with temperature increasing and are close to the Dulong-Petit limit at higher temperatures, Debye temperature decreases with temperature increasing, and increases with pressure rising. The cubic-FezTiA1 is stable mechanically under 250 GPa. Moreover, under lower pressure, thermal expansion coefficient rises rapidly with temperature increasing, and the increasing rate becomes slow at higher pressure.     

First-principles investigation on the structural and elastic properties of cubic-Fe_2 TiAl under high pressures

The structural, elastic, and thermodynamic properties of cubic-Fe 2 TiAl under high temperatures and pressures are investigated by performing ab initio calculation and using the quasi-harmonic Debye model. Some ground state properties such as lattice constant, bulk modulus, pressure derivative of the bulk modulus, and elastic constants are in good agreement with the available experimental results and theoretical data. The thermodynamic properties of Fe 2 TiAl such as thermal expansion coefficient, Debye temperature, and heat capacity in ranges of 0 K-1200 K and 0 GPa-250 GPa are also obtained. The calculation results indicate that the heat capacities at different pressures all increase with temperature increasing and are close to the Dulong-Petit limit at higher temperatures, Debye temperature decreases with temperature increasing, and increases with pressure rising. The cubic-Fe 2 TiAl is stable mechanically under 250 GPa. Moreover, under lower pressure, thermal expansion coefficient rises rapidly with temperature increasing, and the increasing rate becomes slow at higher pressure.     

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.     

Theoretical investigation on the electronic structure,elastic properties, and intrinsic hardness of Si2N20

According to the density functional theory we systematically study the electronic structure, the mechanical prop- erties and the intrinsic hardness of Si2N2O polymorphs using the first-principles method. The elastic constants of four Si2N2O structures are obtained using the stress-strain method. The mechanical moduli （bulk modulus, Young’s mod-ulus, and shear modulus） are evaluated using the Voigt-Reuss-Hill approach. It is found that the tetragonal Si2N2O exhibits a larger mechanical modulus than the other phases. Some empirical methods are used to calculate the Vickers hardnesses of the Si2N2O structures. We further estimate the Vickers hardnesses of the four Si2N2O crystal structures, suggesting all Si2N2O phases are not the superhard compounds. The results imply that the tetragonal Si2N2O is the hardest phase. The hardness of tetragonal Si2N2O is 31.52 GPa which is close to values of β-Si3N4 and γ-Si3N4.     

Electronic structure and elastic properties of scandium carbide and yttrium carbide: A first principles study

We have studied the electronic, structural, and elastic properties of scandium carbide and yttrium carbide by means of accurate first principles total energy calculations using the full-potential linearized plane wave method (FP-LAPW). We have used the generalized gradient approximation (GGA) for the exchange and correlation potential. Volume optimization, energy band structure, and density of states (DOS) of the systems are presented. The second order elastic constants have been calculated and other related quantities such as the Zener anisotropy factor, Poisson's ratio, Young's modulus, Kleinman parameter, Debye temperature, and sound velocities have been determined. The band gap calculation shows that YC is relatively more ionic than ScC.     

First-principles investigation of the effects of strain on elastic thermal,and optical properties of CuGaTe_2

Based on the density functional theory, the influences of strain on structural, elastic, thermal and optical properties of CuGaTe_2 are discussed in detail. It is found that the tensile strain on CuGaTe_2 is beneficial to the decrease of lattice thermal conductivity by reducing the mean sound velocity and Debye temperature. Moreover, all strained and unstrained CuGaTe_2 exhibit rather similar optical characters. But the tensile strain improves the ability to absorb sunlight in the visible range.These research findings can give hints for designing thermoelectric and photovoltaic devices.     

Structural and elastic properties of InX (X = P,As, Sb) at pressure and room temperature

We have investigated the pressure-induced phase transition of InX (X = P, As, Sb) from Zinc-Blende (ZB) to NaCl structure by using realistic interaction potential model involving the effect of temperature. This model consists of Coulomb interaction, three-body interaction and short-range overlap repulsive interaction upto the second nearest neighbor involving temperature. Phase-transition pressure is associated with a sudden collapse in volume, showing the incidence of first-order phase transition. The phase-transition pressure is associated with volume collapses, and the elastic constants obtained from the present model indicate good agreement with the available experimental and theoretical data.     

First-principles study of the structural, elastic, electronic, optical, and vibrational properties of intermetallic Pd2Ga

The structural,elastic,electronic,optical,and vibrational properties of the orthorhombic Pd2Ga compound are investigated using the norm-conserving pseudopotentials within the local density approximation in the frame of density functional theory.The calculated lattice parameters have been compared with the experimental values and found to be in good agreement with these results.The second-order elastic constants and the other relevant quantities,such as the Young’s modulus,shear modulus,Poisson’s ratio,anisotropy factor,sound velocity,and Debye temperature,have been calculated.It is shown that this compound is mechanically stable after analysing the calculated elastic constants.Furthermore,the real and imaginary parts of the dielectric function and the optical constants,such as the optical dielectric constant and the effective number of electrons per unit cell,are calculated and presented.The phonon dispersion curves are derived using the direct method.The present results demonstrate that this compound is dynamically stable.     

Study of phase transformation and elastic properties of ThX (X = N,P, As and Sb) under high-pressure

An improved interaction potential model (IIPM) has been formulated to theoretically predict the pressure induced phase transition, elastic properties and thermophysical properties of thorium monopnictides (ThX; X = N, P, As and Sb). The phase transition pressures and volume drop obtained from this model show a better agreement with the available experimental than theoretical results. We have achieved elastic moduli, anisotropy factor, Poisson's ratio, Kleinman parameter, shear and stiffness constants on the basis of the calculated elastic constants. To know the anharmonic properties, we have also computed the third-order elastic constants, first-order pressure derivatives of second-order elastic constants and thermophysical quantities. Our results are in reasonable agreement with available measured and others reported data which supports the validity of model.     

First-principles study of structural,elastic, lattice dynamical and thermodynamical properties of GdX (X = Bi,Sb)

The results are presented of first-principles calculations of the structural, elastic and lattice dynamical properties of GdX (X = Bi, Sb). In particular, the lattice parameters, bulk modulus, phonon dispersion curves, elastic constants and their related quantities, such as Young's modulus, Shear modulus, Zener anisotropy factor, Poisson's ratio, Kleinman parameter, and longitudinal, transverse and average sound velocities, were calculated and compared with available experimental and other theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacities, Grüneisen parameter and Debye temperatures were predicted in wide pressure (0?50 GPa) and temperature ranges (0–500 K). The plane-wave pseudopotential approach to the density-functional theory within the GGA approximation implemented in VASP (Vienna ab initio simulation package) was used in all computations.     

Theoretical investigation on the electronic structure, elastic properties, and intrinsic hardness of Si2N2O

According to the density functional theory we systematically study the electronic structure, the mechanical prop- erties and the intrinsic hardness of Si2N2O polymorphs using the first-principles method. The elastic constants of four Si2N2O structures are obtained using the stress-strain method. The mechanical moduli (bulk modulus, Young’s mod- ulus, and shear modulus) are evaluated using the Voigt-Reuss-Hill approach. It is found that the tetragonal Si2N2O exhibits a larger mechanical modulus than the other phases. Some empirical methods are used to calculate the Vickers hardnesses of the Si2N2O structures. We further estimate the Vickers hardnesses of the four Si2N2O crystal structures, suggesting all Si2N2O phases are not the superhard compounds. The results imply that the tetragonal Si2N2O is the hardest phase. The hardness of tetragonal Si2N2O is 31.52 GPa which is close to values of β-Si3N4 and γ-Si3N4.     

Electronic band structure,stability, structural,and elastic properties of IrTi alloys

The structural properties and mechanical stabilities of B2-IrTi have been investigated using first-principle calculations. The elastic constants calculations indicate that the B2-IrTi is unstable to external strain and the softening of C₁₁−C₁₂ triggers the B2-IrTi (cubic) to L1₀-IrTi (tetragonal) phase transformation. Detailed electronic structure analysis revealed a Jahn–Teller-type band split that could be responsible for elastic softening and structure phase transition. The cubic–tetragonal transition is accompanied by a reduction in the density of states (DOS) at the Fermi level and the d-DOS of Ti at Fermi level plays a decisive role in destabilizing the B2-IrTi phase.     

Structural stabilities and band structure characteristics of platinum nitride (PtN) via first-principles calculations

The structural phase transformations of the PtN compound with a 1:1 stoichiometric ratio of Pt:N were investigated using the framework of density functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method within the generalized gradient (PBE-GGA) and the Engel–Vosko generalized gradient (EV-GGA) approximations were used. A comparative study of the experimental and theoretical results is provided on the structural properties of zinc-blende (ZB), rock-salt (RS), cesium chloride (CsCl), wurtzite (WZ), nickel arsenide (NiAs), lead monoxide (PbO), and tungsten carbide (WC) phases. The calculated band structure using the modified version of the Becke and Johnson (mBJ) exchange potential reveals the metallic character of the PtN compound. The present study also shows that the PtN compound crystallizes in the WZ phase under ambient conditions. The theoretical transition pressures from WZ to RS, NiAs, PbO, and CsCl transformations are found to be 9.441 GPa, 7.705 GPa, 18.345 GPa and 31.9 GPa, respectively, using the PBE-GGA method.     

An ab initio study of the structural,elastic, electronic and optical properties of the newly synthesized nitridoaluminate LiCaAlN2

The structural parameters, elastic constants, electronic structure and optical properties of the recently reported monoclinic quaternary nitridoaluminate LiCaAlN₂ are investigated in detail using the ab initio plane-wave pseudopotential method within the generalized gradient approximation. The calculated equilibrium structural parameters are in excellent agreement with the experimental data, which validate the reliability of the applied theoretical method. The chemical and structural stabilities of LiCaAlN₂ are confirmed by calculating the cohesion energy and enthalpy of formation. Chemical band stiffness is calculated to explain the pressure dependence of the lattice parameters. Through the band structure calculation, LiCaAlN₂ is predicted to be an indirect band gap of 2.725 eV. The charge-carrier effective masses are estimated from the band structure dispersions. The frequency-dependent dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity coefficient and electron energy loss function spectra are calculated for polarized incident light in a wide energy range. Optical spectra exhibit a noticeable anisotropy. Single-crystal and polycrystalline elastic constants and related properties, including isotropic sound velocities and Debye temperatures, are numerically estimated. The calculated elastic constants and elastic compliances are used to analyse and visualize the elastic anisotropy of LiCaAlN₂. The calculated elastic constants demonstrate the mechanical stability and brittle behaviour of the considered material.     

第一性原理研究Mg2 Si同质异相体的结构、电子结构和弹性性质

在第一性原理框架下,采用赝势平面波方法研究了三种Mg₂Si同质异相体的晶胞结构、电子结构和弹性性质随压强的变化关系.研究发现,反萤石结构Mg₂Si、反氯铅矿结构Mg₂Si和Ni₂In型Mg₂Si分别在压强为0-7 GPa,7.5-20.2 GPa和21.9-40 GPa范围内能够保持结构稳定.计算获得了不同压强下Mg₂Si的弹性常数、体模量、剪切模量、杨氏模量、泊松比和各向异 关键词： 态密度 电子结构 弹性常数 第一性原理     

Study of structural stabilities and optical properties of HgTe under high pressure

A theoretical investigation on the structural stabilities and electronic properties of HgTe under high pressure was conducted using first principles based on density functional theory. Our results demonstrate that the sequence of the pressure-induced phase transitions of HgTe is from the zinc blende, to cinnabar, rocksalt, orthorhombic, and CsCl-type structures. The pressure effects on the optical properties were discussed and compared with previous calculations and experimental data whenever available.     

The stabilities, electronic structures and elastic properties of Rb-As systems

The structural, electronic and elastic properties of Rb–As systems (RbAs in NaP, LiAs and AuCu structures, RbAs2 in the MgCu2 structure, Rb3 As in Na3As, Cu3 P and Li3Bi structures, and Rb5 As4 in the A5B4 structure) are investigated with the generalized gradient approximation in the frame of density functional theory. The lattice parameters, cohesive energies, formation energies, bulk moduli and the first derivatives of the bulk moduli (to fit Murnaghan’s equation of state) of the considered structures are calculated and reasonable agreement is obtained. In addition, the phase transition pressures are also predicted. The electronic band structures, the partial densities of states corresponding to the band structures and the charge density distributions are presented and analysed. The second-order elastic constants based on the stress-strain method and other related quantities such as Young’s modulus, the shear modulus, Poisson’s ratio, sound velocities, the Debye temperature and shear anisotropy factors are also estimated.