Optical and electronic properties of lithium thiogallate (LiGaS₂): experiment and theory

We report the relation between the optical properties and electronic structure of lithium thiogallate (LiGaS₂) by performing XPS and XES measurements and theoretical calculations. According to the XPS measurements, the LiGaS₂ crystals grown by the Bridgman-Stockbarger method possess promising optical qualities, low hygroscopicity and high stability upon middle-energy Ar⁺-ion irradiation. The difference in the LiGaS₂ band gaps obtained by theoretical calculations and experimental measurements was, for the first time, reduced down to 0.27 eV by applying the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential where the Coulomb repulsion was considered by introducing Hubbard parameter, U. The TB-mBJ+U method also reproduces the XPS spectrum well. The TB-mBJ+U band-structure calculations of LiGaS₂ are found to be in good agreement with the XPS and XES experimental data. The accurate electronic structure of LiGaS₂ allows further investigation of the optical properties. The relation between the photoluminescence of LiGaS₂ and its electronic structure was revealed. Moreover, the theoretical results show the possibility of emissions at higher energy levels in LiGaS₂, that has not been measured in experiments yet. Good phase-matching of LiGaS₂ was expected to occur at energy levels of 5, 6, 6.2, 7, 7.2, and 8 eV.