The linear optical properties for NaCl phase of calcium mono chalcogenides by density functional theory

In this study, a complete theoretical analysis of optical properties of calcium mono chalcogenide compounds CaX (X = O, S, Se and Te) in NaCl crystal structure are calculated using the band structure results obtained through 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 reflectivity R(ω) and the energy loss function are calculated by 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, those due to single particle transitions, while excitonic peaks are completely missing. Furthermore the interband transitions responsible for the structures in the spectra are specified. It is shown that the chalcogen p states and Ca 3d states play the major role in optical transitions as initial and final states, respectively. The effect of the spin-orbit coupling on the optical properties is also investigated and found to be quite small, especially in the low energy region. The dielectric constants are calculated and compared with the available theoretical and experimental results for CaO, CaS and CaSe while, for CaTe the dielectric constant is not available.