First-principles study on the structural, electronic, and optical properties of Ca1−xSrxSe alloys

The structural, electronic, and optical properties of binary CaSe and SrSe compounds and Ca1−xSrxSe alloys were studied by using the full potential linearized augmented plane wave (FPLAPW) method within density functional theory (DFT). The band structure calculations showed that the CaSe and the SrSe binary compounds in the rocksalt (RS), zinc-blende (ZB) and wurtzite (WZ) phases were semiconductors while they had a metallic characteristic in the CsCl phase. The lattice constant and bulk modulus values for the Ca1−xSrxSe alloys in the RS and the ZB phases at different concentrations were calculated and compared with those obtained by using Vegard’s law. The energy band gap values in the RS and the ZB phases were estimated for different x values by using both define acronyms the Perdew, Burke, and Ernzerhof (PBE-GGA) and the Engel and Vosko (EV-GGA) schemes, and the results were compared with those obtained by using the empirical electronegativity expression. The band gap bowing parameters were calculated by using quadratic functions and the procedure of Bernard and Zunger to fit the non-linear variation of the band gaps. The static dielectric constant ε1(0) was calculated at different concentrations. The energy loss function L(ω) for the Ca1−xSrxSe alloys in the RS and the ZB phases has a main peak corresponding to the plasmon frequency. The values of the static refractive index (n(0)) for the Ca1−xSrxSe alloys were calculated and compared with the values predicted by using the Moss, Ravindra, and Vandamme models. Finally, the extinction indic incident photon energies. es (k(ω)) and the reflectivities (R(ω)) for the Ca1−xSrxSe alloys were calculated within a wide range of incident photon energies.