Etude ab initio des propriétés élastiques, électroniques, optiques et thermodynamiques des composés BeP2N4 et Si3N4 pouvant avoir la structure spinelle

Abstract

Using the pseudopotential plane wave (PPW) method based on the density functional theory (DFT) with the local density approximation functional (LDA) implemented CASTEP package, an ab initio study of the structural, elastic, electronic, optical and thermodynamic properties of compounds BeP2N4 and Si3N4 has been realised. Calculated lattice parameters, structural stability and phase transition pressure (phenakite, hexagonal and spinel) of both polymorphs of these compounds have been treated and are in good agreement with available data. Pressure dependence of the single-crystal and polycrystalline elastic moduli and their related properties has been investigated. Thermal conductivity minimums for both BeP2N4 and Si3N4 polymorphs have been estimated through Clarke's and Cahill's models. Elastic anisotropy has been analyzed for both considered polymorphs. Indirect electronic fundamental band gaps ᴧ ‒ Γ and  ‒ Γ have been deduced for BeP2N4 and Si3N4 respectively. Dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and energy loss function have been calculated as function of frequency. Employing the quasi-harmonic Debye model (QHDM), pressure and temperature dependence of some macroscopic thermodynamic parameters have been estimated and predicted for the first time for both phases of the title compound BeP2N4.