DFT study of structural,electronic and elastic properties of two polymorphs of monoclinic CsGaQ2 (Q = S,Se)

In this paper, we report an ab-initio calculations of the structural, electronic and elastic properties of monoclinic CsGaQ₂ (Q?=?S, Se) crystals in two polymorphs CsGaQ₂-mC64 and CsGaQ₂-mC16 (Q?=?S, Se). The investigation is done using the pseudo-potential plane-wave (PP-PW) method combined to the generalized gradient approximation (GGA) within the density functional theory (DFT). The calculated equilibrium lattice constants (a, b and c), angle β are in good agreement with the available experimental data. We have calculated and analyzed the energy gap, band structure and density of states. The electronic structure calculation demonstrates that crystals are direct-gap semiconductors. The single-crystal elastic constants C_ij of CsGaQ₂-mC16 are predicted, for the first time, using the stress–strain method. The polycrystalline bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio?ν, and elastic anisotropy A^U are determined based on the predicted C_ij. Our results indicate that CsGaQ₂ (Q?=?S, Se) can be classified as brittle materials.     

Study of elastic properties of CeO2 by statistical moment method

The purpose of the present paper is to investigate the temperature and pressure dependences of the elastic properties of cerium dioxide using the statistical moment method (SMM). The equation of states of bulk CeO₂ is derived from the Helmholtz free energy, and the pressure dependences of the elastic moduli like the bulk modulus, B_T, shear modulus, G, Young’s modulus, E, and elastic constants (C11, C12, and C44) are presented taking into account the anharmonicity effects of the thermal lattice vibrations. In the present study, the influence of temperature and pressure on the elastic moduli and elastic constants of CeO₂ has also been studied, using three different interatomic potentials. We compare the results of the present calculations with those of the previous theoretical calculations as well as with the available experiments.     

Thermodynamic,elastic, elastic anisotropy and minimum thermal conductivity of β-GaN under high temperature

The thermodynamic, elastic, elastic anisotropy and minimum thermal conductivity of β-GaN are investigated at ambient pressure and high temperature by using first-principles calculations method with the ultrasoft psedopotential scheme. The elastic constants calculations reveal β-GaN is mechanically stability at ambient pressure and high temperature. The elastic modulus (Poisson's ratio, shear modulus and Young's modulus) decreases with increasing temperature. The calculations of anisotropy show that β-GaN has a larger elastic anisotropy in Poisson's ratio, shear modulus, Young's modulus and Zener anisotropy index. In addition, when the temperature increases from 0 to 1500 K, the elastic anisotropy decreases for β-GaN. The quasi-harmonic Debye model is successfully applied to determine the thermodynamic properties at different pressures and temperatures. Using the quasi-harmonic Debye model, the thermodynamic properties including the Debye temperature, Grüneisen parameter, the heat capacity, adiabatic bulk modulus, and the thermal expansion coefficients of β-GaN are predicted under high temperature and high pressure.     

Theoretical investigation of new type of ternary magnesium alloys AMgNi4 (A=Y,La, Ce,Pr and Nd)

New ternary magnesium alloys AMgNi₄ (A=Y, La, Ce, Pr and Nd) have been studied by First-Principles calculations within the generalized gradient approximation. The optimized structural parameters were in good agreement with the available experimental data. The calculated cohesive energies and formation enthalpies showed that these alloys had strong structural stability. Then the elastic constants C_ij of these AMgNi₄ alloys were calculated, and the bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio ν and anisotropy value A of polycrystalline materials were derived from the elastic constants, the related mechanical properties were further discussed. The electronic structures were also calculated to reveal the underlying mechanism for the structural stability and the elastic property.     

First-principles calculations of structural stability and mechanical properties of tungsten carbide under high pressure

The structural stability and mechanical properties of WC in WC-, MoC- and NaCl-type structures under high pressure are investigated systematically by first-principles calculations. The calculated equilibrium lattice constants at zero pressure agree well with available experimental and theoretical results. The formation enthalpy indicates that the most stable WC is in WC-type, then MoC-type finally NaCl-type. By the elastic stability criteria, it is predicted that the three structures are all mechanically stable. The elastic constants C_ij, bulk modulus B, shear modulus G, Young?s modulus E and Poisson?s ratio ν of the three structures are studied in the pressure range from 0 to 100 GPa. Furthermore, by analyzing the B/G ratio, the brittle/ductile behavior under high pressure is assessed. Moreover, the elastic anisotropy of the three structures up to 100 GPa is also discussed in detail.     

Elastic,electronic and thermodynamic properties of fluoro-perovskite KZnF3 via first-principles calculations

The elastic, electronic and thermodynamic properties of fluoro-perovskite KZnF₃ have been calculated using the full-potential linearized augmented plane wave (FP-LAPW) method. The exchange-correlation potential is treated with the generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE). Also, we have used the Engel and Vosko GGA formalism (GGA-EV) to improve the electronic band structure calculations. The calculated structural properties are in good agreement with available experimental and theoretical data. The elastic constants C _ij are calculated using the total energy variation with strain technique. The shear modulus, Young’s modulus, Poisson’s ratio and the Lamé coefficients for polycrystalline KZnF₃ aggregates are estimated in the framework of the Voigt-Reuss-Hill approximations. The ductility behavior of this compound is interpreted via the calculated elastic constants C _ij . Electronic and bonding properties are discussed from the calculations of band structure, density of states and electron charge density. The thermodynamic properties are predicted through the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variation of bulk modulus, lattice constant, heat capacities and the Debye temperature with pressure and temperature are successfully obtained.     

The mechanical,thermodynamic and electronic properties of Al3Nb with DO22 structure: A first-principles study

The lattice constants, elastic properties, electronic structure and thermodynamic properties of Al₃Nb with DO₂₂ structure have been investigated by the first-principles calculation. The calculated lattice constants were consistent with the experimental values, and the structural stability was also studied from the energetic point of view. The single-crystal elastic constants (C_ij) as well as polycrystalline elastic parameters (bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio υ and anisotropy value A) were calculated, and brittleness of Al₃Nb was discussed in detail. Besides, the electronic structure of tetragonal Al₃Nb was studied, which indicates a mixture of metallic bond and covalent bond in Al₃Nb and reveals the underlying mechanism of the stability and elastic properties of Al₃Nb. Finally, the thermodynamic properties of Al₃Nb were calculated and the physical properties such as heat capacity and Debye temperature were predicted within the quasi-harmonic approximation.     

Theoretical investigations of NiTiSn and CoVSn compounds

The structural, elastic and electronic properties of NiTiSn and CoVSn half-Heusler compounds have been calculated using the full-potential linear muffin-tin orbital (FP-LMTO) method. The computed equilibrium lattice constants are in excellent agreement with the available experimental and theoretical data. The elastic constants C_ij are calculated using the total energy variation with strain technique. The polycrystalline elastic moduli (namely: the shear modulus, Young's modulus, Poisson's ratio, Lamé's coefficients, sound velocities and the Debye temperature) were derived from the obtained single-crystal elastic constants. The ductility mechanism for the studied compounds is discussed via the elastic constants C_ij and their related parameters. The electronic band structure calculations show that the conduction band minimum (CBM) is located at the X point for both compounds, whereas the valence band maximum (VBM) is located at the Г point for NiTiSn and at the L point for CoVSn, resulting in indirect energy band gaps of 0.46 and 0.75 eV for NiTiSn and CoVSn, respectively. The pressure and volume dependences of the energy band gaps have been calculated.     

Pressure effect on structural,electronic and thermodynamic investigations of europium filled skutterudite EuFe4Sb12: LDA and LSDA approximations

Numerical calculations based on the full potential muffin-tin orbitals method (FP-LMTO) within the local density approximation (LDA) and the local spin-density approximation (LSDA) to investigate the structural, electronic and thermodynamic properties of filled skutterudite EuFe₄Sb₁₂ are presented. The electronic band structure and density of states profiles prove that this material is a conductor. The present investigation is also extended to the elastic constants, such as the bulk modulus B, anisotropy factor A, shear modulus G, young's modulus E, Poisson's ratio ν, and the B/G ratio with pressure in the range of 0–40 GPa. The sound velocities and Debye temperatures are also predicted from the above constants. The variations of the primitive cell volume, expansion coefficient α, bulk modulus B, heat capacity (C_p and C_v), Debye temperature θ_D, Helmholtz free energy A, Gibbs free energy G, entropy S, and internal energy U with pressure and temperature in the range 0–3000 K are calculated successfully.     

First-principles investigations on elastic, phonon and thermodynamic properties of SrB6 under pressure

The elastic, phonon and thermodynamic properties of the divalent alkaline-earth hexaboride SrB₆ are investigated by using plane-wave pseudopotential density functional theory method. The calculated structure parameters and bulk modulus are well consistent with the available experiment and theoretical data. The pressure dependences of elastic constants C_ij, bulk modulus B₀, shear modulus G, Young's modulus E and Poisson's ratio σ are also presented. With these elastic parameters, we investigate the mechanical stability and compressibility of SrB₆. For the thermodynamic properties, both phonon and quasi-harmonic Debye model methods are adopted. Through the comparison with experimental and other theoretical results, we found the method of quasi-harmonic Debye model is a little better. Moreover, the phonon dispersion relations are also obtained. It is found that there are two LO/TO splitting around 5 THz and 26 THz, respectively.     

Determination of the elastic constants of copper on copper-whiskers by the flexural vibration method

Copper-whiskers with different orientations were excited in flexural vibrational modes. From the resonance frequencies and the cubic compressibility, the elastic constants c_ik of copper were determined. The results are in good agreement with values obtained by other authors using other methods with macroscopic single crystals of copper. No enhancement of Young's modulus, reported recently in thin metallic films, was seen in Cu-whiskers.     

The first principles study on the Boron antimony compound

We present the results of our calculations on Boron antimony (BSb) compound in zinc-blende (ZB) and rock-salt (RS) structures by performing ab initio calculations within the local density approximation (LDA). Some basic physical properties, such as lattice constant, bulk modulus, cohesive energy, phase transition pressure, second-order elastic constants (C_ij), phonon frequencies, and some band structural parameters are calculated and compared with those obtained with other recent theoretical works. In order to further understand the behaviour of BSb compound, we have also predicted, the pressure-dependent behaviours of the band gap, second-order elastic constants (C_ij), Young's modulus, poison ratios (ν), Anizotropy factor (A), sound velocities, and Debye temperature for this hypothetical compound.     

The elastic behaviour of tetrahedral materials with vacant sites

As part of a study of defect tetrahedral structure compounds, the elastic constants of single crystal specimens of Hg₃Ga₂□Te₆ and HgIn₂□Te₄ have been measured between 77 K and room temperature using the pulse superposition technique. These compounds are included in the series HgTe, Hg₅In₂□Te₈, Hg₃In₂□Te₆ and HgIn₂□Te₄ and HgTe, Hg₅Ga₂□Te₈, Hg₃Ga₂□Te₆ where there is a progressive increase in the concentration of vacant sites. While the other compounds studied are cubic, HgIn₂□Te₄ has a tetragonal structure with a $\text{c}\text{a}$ ratio of 2.0. The components of the elastic stiffness tensor of this material at 77 K are (in units of 10¹¹ dyne cm^?2) C₁₁ = 4.31 C₁₂ = 2.54 C₄₄ = 2.14 C₃₃ = 4.47 C₁₃ = 2.18 C₆₆ = 2.41. In a cubic material C₁₁ = C₃₃, C₄₄ = C₆₆ and C₁₂ = C₁₃: the elastic behaviour of this tetragonal compound closely resembles that of a cubic material, as might be anticipated from its structure. This similarity is further illustrated by reference to the symmetry of phase velocity and Young's modulus surfaces. Examination of the elastic constants and reduced elastic constants of these compounds shows a regular trend, the elastic stiffness decreases as the number of vacant sites increases. There is an approximately linear relationship between the reduced bulk modulus and the number of sited vacancies.     

Theoretical investigations of structural, elastic and thermodynamic properties for PtN2 under high pressure

We have investigated structural and elastic properties of PtN₂ under high pressures using norm-conserving pseudopotentials within the local density approximation (LDA) in the frame of density-functional theory. Calculated results of PtN₂ are in agreement with experimental and available theoretical values. The a/a₀, V/V₀, ductility/brittleness, elastic constants C_ij, shear modulus C′, bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio σ and anisotropy factor A as a function of applied pressure are presented. Through the quasi-harmonic Debye model, we also study thermodynamic properties of PtN₂. The thermal expansion versus temperature and pressure, thermodynamic parameters X (X=Debye temperature or specific heat) with varying pressure P, and heat capacity of PtN₂ at various pressures and temperatures are estimated.     

The second-order elastic constants of AgBr from 77 to 300 K

The three independent second-order elastic constants of AgBr have been measured from room temperature down to liquid nitrogen temperature A single crystal with a (110) axis was used for the measurements The measured longitudinal elastic constant C'₁₁ = (C₁₁ + C₁₂ + 2C₄₄)/2 increases by 16% over this temperature range The elastic shear constant C₄₄) increases by almost 11%, while the elastic shear constant C' = (C₁₁ ? C₁₂)/2 increases by 50% over this range The measured bulk modulus B = (C₁₁) + 2C₁₂)/3 increases by almost 15% as the lattice becomes stiffer The initial temperature derivatives of the elastic constants are similar to those previously measured The changes in the elastic constants are basically linear down to approximately 150 K, where the temperature derivatives begin to decrease in magnitude These results are similar to those previously obtained for AgCl     

Elastic properties of mono- and polycrystalline ( and Lu) under pressure effect

The structural and elastic properties of perovskite-type RCRh₃, with R=Sc, Y, La and Lu, under pressure effects have been investigated using the pseudo-potential plane-wave method based on the density functional theory within the generalized gradient approximation. For monocrystalline RCRh₃, the optimized lattice constants, elastic constants and directional elastic wave velocities are calculated and analyzed in comparison with the available experimental and theoretical data. An increase in the lattice constant has been found with increasing atomic size of the R element and a corresponding decrease in the hardness. The anisotropic elastic constants and directional elastic wave velocities increase linearly with increasing pressure. A set of elastic parameters and related properties, namely bulk and shear moduli, Young’s modulus, Poisson’s ratio, Lamé’s coefficients, average sound velocity and Debye temperature are predicted in the framework of the Voigt–Reuss–Hill approximation for polycrystalline RCRh₃. We have found that the toughness of RCRh₃ compounds can be improved at high pressure.     

Concentration dependences of the elastic constants of the two-dimensional graphene-silicene system

The concentration dependences of the elastic constants of the two-dimensional Si _x C_{1 − x} system have been investigated with the use of the Harrison bonding-orbital method and the Keating model. The central and non-central force constants and the Grüneisen parameter have been considered by means of the bonding-orbital method. All quantities under consideration have been shown to exhibit a nonlinear behavior during the transition from graphene to silicene. A nontrivial role of the short-range repulsion has been discussed. The second-order and third-order elastic constants, the pressure dependences of the second-order elastic constants, as well as the Poisson’s ratio and Young’s modulus have been investigated in the Keating model. It has been found that the elastic constants and Young’s modulus change almost linearly upon the transition from graphene to silicene, whereas the other quantities under consideration exhibit nonlinearity.     

Elastic and thermodynamic properties of AVO3 (A=Sr, Pb) perovskites

We have investigated the elastic and thermodynamic properties for the perovskite type metavanadate SrVO₃ and the multiferroic PbVO₃, probably for the first time by the means of a Modified Rigid Ion Model (MRIM). We present the elastic constants (C_11,C_12,C₄₄) and other elastic properties like Bulk modulus (B), Young′s modulus (E), shear modulus (G), Poisson′s ratio (σ) and wave velocity (υ_l, υ_s, υ_m). Besides we have reported the thermodynamic properties molecular force constant (f), Reststrahlen frequency (ν), cohesive energy (?), Debye temperature (θ_D) and Gruneisen parameter (γ). We have also computed the variation of heat capacity (C_P) and there by volume thermal expansion coefficient (α) in a wide temperature range. We found that the computed properties reproduce well with the available data in literature. To our knowledge some of the properties are reported for the first time.     

First-principles investigation of the elastic and electronic properties of the binary intermetallics in the Al–La alloy system

The structural, elastic and electronic properties of Al₂La, AlLa₃ and Al₃La binary intermetallics in the Al–La alloy system were investigated using the first-principles method. The calculated lattice constants were consistent with the experimental values. Formation enthalpy and cohesive energy showed that the studied Al₂La, AlLa₃ and Al₃La all have a higher structural stability, and the alloying ability of Al₂La and Al₃La is stronger than that of AlLa₃. The single-crystal elastic constants (C_ij) as well as polycrystalline elastic parameters (bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio υ and anisotropy value A) were calculated by the Voigt–Reuss–Hill (V–R–H) approximations, and the relationship of these elastic parameters between Al₂La, AlLa₃ and Al₃La phases were discussed in detail. The results showed that Al₂La and Al₃La which are anisotropic materials are absolutely brittle, while the isotropic AlLa₃ is slightly ductile. Finally, the electronic density of states (DOS) was also calculated to reveal the underlying mechanism of structural stability.