First principles calculations of the structural, electronic and optical properties of the mixed fluorides SrxCd1−xF2

In the present work, we have investigated the structural, electronic and optical properties of SrF2 and CdF2 and their ternary mixed SrxCd1−xF2 alloys at some selected concentrations x (x = 0.25, 0.50, 0.75 and 1). The parent compounds SrF2 and CdF2 crystallize in Fm-3 m space group, whereas the alloys adopt the cubic structure with Pm-3 m space group for the composition x = 0.25 and 0.75 and the tetragonal structure with space group P4/mmm for x = 0.50. The calculations were performed using the full-potential linearized augmented plane wave (FP-LAPW) method. The exchange-correlation potential was handled with Wu and Cohen GGA approximation (WC-GGA). Moreover, the Engel-Vosko's (EV-GGA) formalism and the modified Becke Johnson (mBJ) approximation were also applied to improve the electronic band structure calculations. The computed structural parameters for SrF2 and CdF2 such as the equilibrium lattice constants and the bulk moduli are in good agreement with the available experimental and theoretical data. It is found that the lattice parameters increase with increasing composition (x) while the bulk modulus decreases for SrxCd1−xF2 alloys. The calculated band structures reveal an indirect band gap (W-Γ), (X-Γ) and (M-Γ) for CdF2, SrF2 and SrxCd1−xF2 for x = 0.25, 0.75 and x = 0.5, respectively. The optical constants, including the dielectric function, refractive index, reflectivity, absorption, extinction coefficient and the energy loss function were calculated using both WC-GGA and mBJ schemes for a radiation up to 40 eV. This is the first quantitative theoretical prediction of the optical properties for these alloys that requires experimental confirmation.