Single crystals of undoped Li2WO4 and Li2W1-0.0125Mo0.0125O4: formation enthalpies, heat capacity in the temperature range 320-997 K.

The Li2WO4 single crystal was first grown by applying the Czochralski technique with weight control and low-temperature-gradients. A single crystal of Li2WO4 is one of the perspective materials for researching rare events. The heat capacity for a Li2W1-0.0125Mo0.0125O4 single crystal has been determined by DSC calorimetry in the temperature range 320-997 K for the first time. No anomalies in the heat capacity associated with phase transitions were found. The standard formation enthalpy for the Li2WO4 single crystal was studied using reaction calorimetry. It has been shown that the relation of standard formation enthalpies for Li2W1-xMoxO4 (x = 0.15-0) with function (1-x)W + xMo are close to linear, which allows one to predict the thermodynamic properties for single crystals with isotopes Li2W1861-0.0125Mo1000.0125O4 and Li2W186O4. It was shown that single crystals with isotopes are more thermodynamically stable than without isotopes.

Thermophysical properties of lithium thiogallate that are important for optical applications.

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-x S 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.