FP-APW+lo calculations of the electronic and optical properties of alkali metal sulfides underpressure

Abstract

The electronic and optical properties ofM2S (M = Li,Na, K and Rb) compounds in the cubic antifluorite structure have been calculated, using afull relativistic version of the full-potential augmented plane-wave plus local orbitalsmethod based on density functional theory, within both the local density approximation(LDA) and the generalized gradient approximation (GGA). Moreover, the Engel–VoskoGGA formalism (EV-GGA) is applied so as to optimize the corresponding potential forband structure calculations. The calculated equilibrium lattices and bulk moduli arein good agreement with the available data. Band structure, density of states,electron charge density and pressure coefficients of energy gaps are given. Resultsobtained for band structure using EV-GGA are larger than those with LDA andGGA. It is found that the spin–orbit coupling lifts the triple degeneracy at theΓ point and the double degeneracy at the X point. The analysis of the electron charge densityshows that the M–S bonds have a significant ionic character. The complex dielectric functionsε2(ω) for alkali metal sulfides were calculated for radiation up to 30 eV and the assignment of thecritical points to the band structure energy differences at various points of the Brillouinzone was made. The pressure and volume dependence of the static dielectric constant andthe refractive index are calculated.