Thermophysical properties of lithium thiogallate that are important for optical applications

DOI: 10.1039/D1RA05698K 

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 LiGaO2−xSx 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 VS 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.