First principles study of the optical properties of alkaline-earth metal nitrides

A detailed analysis of the optical properties of alkaline-earth metal nitrides (Be3N2, Mg3N2 and Ca3N2) has been performed, using the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory. The exchange correlation potential is treated by the generalized gradient approximation within Perdew et al. scheme. The real and imaginary parts of the dielectric function ϵ(ω), the optical absorption coefficient I(ω) the refractive index n(ω), the extinction coefficient k(ω) and the electron energy loss function are calculated within the random phase approximation (RPA). The calculated results show a qualitative agreement with the available experimental results in the sense that we can recognize some peaks qualitatively, that is, ones that are due to single particle transitions. Furthermore, the interband transitions responsible for the structures in the spectra are specified. The metal s states and nitrogen p states play the major role in these optical transitions as initial and final states, respectively, for Mg3N2 and Be3N2. In the case of Ca3N2, where Ca has d levels lying near the Fermi level, the Ca d states are mostly final states. The effect of the spin-orbit coupling on the optical properties is also investigated, and it is found to be quite small, especially in the low energy region. The dielectric constants are calculated and compared with the available experimental results.