First-principles calculations of structural, elastic, electronic and optical properties of XO (X=Ca, Sr and Ba) compounds under pressure effect

The structural, elastic, electronic and optical properties of XO (X= Ca, Sr and Ba) compounds were investigated by the density functional theory. A good agreement was found between our calculated results and the available theoretical and experimental data of the lattice constants. Young's modulus, Poisson ratio, bulk modulus, elastic constants and their pressure derivatives are also calculated. SrO and BaO compounds present a transition phase at 39.72 and 27.28 GPa. The SrO compound shows a change from direct band gap (Γ–Γ) to indirect band gap (Γ–X) at about 15 GPa. The top of the valence bands reflects the s electronic character for all structures. We investigate the effective mass of electrons as function of pressure at the Γ point for CaO, SrO and BaO compounds. Calculations of the optical spectra have been performed for the energy range 0–60 eV. The origin of the spectral peaks was interpreted based on the electronic structures. The enhancement of pressure increases the static dielectric function and refractive index of CaO, SrO and BaO.