Structural and elastic properties of Ba 2 P 7 X (X=Cl, Br, I) ( Barium Phosphide Halides) Zintl compounds have been investigated using the pseudo-potential plane-wave (PP-PW) method based on the density functional theory (DFT) within the generalized gradient approximation (GGA-PBESOL). The calculated lattice constants and internal parameters are in a good agreement with the experimental results reported in literature. In this paper, we present an investigation of the relative changes of the structural parameters and elastic constants as function of hydrostatic pressure. Isotropic elastic moduli and their related properties for single-crystal and polycrystalline phase, including the namely bulk modulus, shear modulus, Young's modulus, Poisson's ratio, elastic anisotropy indexes, Pugh's indicator of brittle/ductile behavior, elastic wave velocities and Debye temperature have been estimated from C i j using Voigt, Russ and Hill approximations. Two different methods have been used to study the elastic anisotropy of these compounds.M. Radjai et al.(barium and indium phosphide) and analyzed its crystalline structure. According to [1], both compounds Ba 2 P 7 Br and Ba 2 P 7 I are crystallized in a new type of monoclinic structure in space group P2 1 /m (No. 11) and are isostructural to Ba 2 P 7 Cl [1]. The crystal structures of Ba 2 P 7 X (X = Cl, Br, I) exhibit the presence of P −3 7 groups with halogenated anions and barium cations. Authors think that neither theoretical nor experimental studies of the elastic properties had been carried out. Therefore, such calculations are made in the present work with the inclusion of pressure effects. The results reported in this paper may be useful for evaluating the potential technological applications of Ba 2 P 7 X. Knowledge of the elastic constants of crystalline materials is essential to understand many of their basic physical properties. In particular, these constants provide information on the stability and stiffness of the material against externally applied stresses [8]. Knowledge of the pressure dependence of elastic constants and lattice parameters is significant for many modern technologies [8,9]. Computational detailsCurrently, there are different theoretical calculation codes with different approximations. In our calculations, we use the code CASTEP (CambridgeSerial Total Energy Package) [10] which is a direct application of the calculation. All calculations were performed using pseudo-potential plane wave ab initio (PP-PW) method based on the density function (DFT). To determine the structural parameters and elastic moduli of the considered compounds, there was used a new version of the generalized gradient approximation (GGA), namely the GGA-PBEsol [11], which has been developed specifically to improve the description of the exchange-correlation in solids. In all electronic total energy calculations, an ultrasoft Vanderbilt pseudo-potential [12] was used to treat the potential seen by the valence electrons due to the nucleus and electrons of the frozen nucleus. B...

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First-principles calculations using the plane-wave pseudopotential method within the generalized gradient approximation method were performed to study the pressure dependence of the structural, elastic, electronic and optical properties for the half-Heusler compounds ScPtSb and YPtSb in a cubic MgAgAs-type structure. The calculations were performed with the inclusion of spin-orbit coupling. The calculated equilibrium lattice parameters are in good agreement with the available experimental and theoretical values. The crystal rigidity and mechanical stability were discussed using the elastic constants and related parameters, namely bulk modulus, shear modulus, Debye temperature, Poisson's coefficient, Young's modulus and isotropic sound velocities. The calculated electronic band structures show that ScPtSb has an indirect gap of Γ-X type, whereas YPtSb has a direct band gap of Γ-Γ type. Furthermore, the effect of pressure on the optical properties, namely the dielectric function, absorption spectrum, refractive index, extinction coefficient, reflectivity and energy-loss spectrum is investigated for both compounds ScPtSb and YPtSb.

In this paper, we performed a detailed theoretical study of structural, elastic and electronic properties of two germanides LuAuGe and ScAuGe by means of first-principles calculations using the pseudopotential plane-wave method within the generalized gradient approximation. The crystal lattice parameters and the internal coordinates are in good agreement with the existing experimental and theoretical reports, which proves the reliability of the applied theoretical method. The hydrostatic pressure effect on the structural parameters is shown. The monocrystalline elastic constants were calculated using the stress-strain technique. The calculated elastic constants of the MAuGe (M = Lu, Sc) compounds meet the mechanical stability criteria for hexagonal crystals and these constants were used to analyze the elastic anisotropy of the MAuGe compounds through three different indices. Polycrystalline isotropic elastic moduli, namely bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and the related properties are also estimated using Voigt-Reuss-Hill approximations. Finally, we studied the electronic properties of the considered compounds by calculating their band structures, their densities of states and their electron density distributions.