First-principles calculations of the structural, elastic, electronic and optical properties of orthorhombic LiGaS2 and LiGaSe2

The structural, electronic and optical properties of two orthorhombic crystals, LiGaS2 and LiGaSe2, were calculated using the density functional theory. The optimized unit cells are in good agreement with experimental data. The mechanical stability of LiGaS2 and LiGaSe2 has been confirmed by calculation of the elastic constants. The band structures, density of states were obtained. It is shown that the two crystals belong to the semiconductors with a direct energy band gap of about 3.12 and 2.34 eV. The bonding properties were analyzed according to the electronic structural data. The optical properties including the dielectric spectra, absorption spectra, refractive index, extinction coefficient, reflectivity and energy-loss spectra were also calculated and the results indicate that the two compounds are promising mid-IR crystal materials.