Optical properties of alkaline-earth fluorohalides BaFX (X = Cl, Br, I) compounds

The ab initio calculations of the optical properties for the alkaline-earth fluorohalides BaFX (X = Cl, Br, I) compounds in its matlockite-type structure have been reported here by using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed generalized gradient approximation (GGA), which is based on exchange–correlation energy optimization to calculate the total energy. Also we have used the Engel–Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that these compounds have a direct energy band gap (Γ–Γ). We have calculated the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε1(0). We find that the values of ε1(0) increases with decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data. The optical properties are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure. A considerable anisotropy is found between the parallel and perpendicular components of the frequency-dependent optical properties.