Band gap engineering and optical response of the ACr2S4 (A=Fe, Co) normal spinels using PBE+U and TB-mBJ potentials

The electronic and optical properties of the ACr2S4 (A=Fe, Co) normal spinels have been studied by use of the full-potential linearized augmented plane wave (FPLAPW) method based on density functional theory (DFT). The calculations were performed by two methods, namely, PBE+U and TB-mBJ, with the addition of spin-orbit coupling in both the schemes. The lattice constant and internal parameter of unit cell were optimized. Band structure obtained are analysed based on Density of States (DOS). We found that more pronounced splitting of A-d and Cr-d states in TB-mBJ scheme is responsible for the larger band gaps. The octahedral field surrounding Cr-atoms splits the Cr-d levels into t2g and eg states with some mixing between the two states. The calculated values of spin magnetic moment per formula unit are consistent with earlier report. Optical properties are calculated along the directions of lattice constants to analyze their anisotropic nature based on energy level transitions. We also studied the refractive index, n (ω), and the extinction coefficient, k (ω), by PBE+U and TB-mBJ scheme.