Thermophysical properties of lithium thiogallate that are important for optical applications

Abstract

Lithium thiogallate LiGaS₂ is one of the most common nonlinear crystals for mid-IR due to its extreme beam strength and wide transparency range; however, its thermophysical properties have not yet been practically studied. Large crystals of high optical quality are grown. DTA revealed features at 1224 K below melting point (1304 K) that are associated with the oxygen containing compounds of the LiGaO_2−xS_x type. The thermal conductivity of LiGaS₂ (about 10.05 W (m⁻¹ K⁻¹)) and band gap value (3.93 eV at 300 K) are found to be the highest in the LiBC₂ family. Isotropic points in the dispersion characteristics for the refractive index are found and LiGaS₂-based narrow-band optical filters, smoothly tunable with temperature changes, are demonstrated. Intense blue photoluminescence of anionic vacancies V_S is observed at room temperature after annealing LiGaS₂ in vacuum, whereas orange low-temperature emission is related to self-trapped excitons. When LiGaS₂ crystals are heated, spontaneous luminescence (pyroluminescence) takes place, or thermoluminescence after preliminary UV excitation; the parameters of traps of charge carriers are estimated. The obtained data confirm the high optical stability of this material and open up prospects for the creation of new optical devices based on LiGaS₂.