The first-order Raman spectrum of K₂S was measured over the temperature range from 10 to 742 K. The temperature dependence of the linewidth can be explained using results from the anharmonic lattice dynamics approach. Both the cubic and the quartic anharmonic interactions are of importance for this system. At 293 K, the Raman line is at ω_T2g=128 cm^−1 with a full width at half maximum Γ_T2g=2.9 cm^−1.

Li2S, cubic, Fiiβm (No. 225), a = 5.7158(1) A, V= 186.7 A3, Z = 4, REi(F) = 0.009, Rv.(F) = 0.009, T= 293 K.

Experiments in which the Raman linewidth was measured as a function of temperature (7-1183 K) and pressure (0-400 bar) were performed on the (111) and (100) planes of single crystals of the cubic anti-fluorite Li₂S. The temperature dependence of the lattice constant was determined by x-ray diffraction (11-295 K). From these results and published Brillouin scattering data for this host, the volume thermal expansion coefficient as a function of temperature was obtained as well as the isothermal compressibility and the isothermal Raman mode Grüneisen parameter. Using the thermodynamic approach within the quasi-harmonic approximation, we show that below 400 K the volume effects describe well the temperature dependence of the Raman linewidth whereas above this temperature there are direct anharmonic effects appearing. Above approximately 850 K new Raman lines appear that are A_⊥ and E polarized.

We study the ionic conductivity versus temperature and frequency of large Na²S single crystals by using a calibrated impedance apparatus. The experimental setup used for the ionic conductivity measurements up to 1350 K and its calibration are described. The apparatus allows to measure complex impedances between 0.1 Ω and 10 GΩ. The high temperature conductivity data were analyzed in terms of the conventional Frenkel defect model. We assume that cation vacancies and cation interstitials are the dominant intrinsic defects. The energy of motion was found to be 0.61 ± 0.05 eV for a cation vacancy. The energy of formation of a Frenkel defect pair is 2.51 ± 0.05 eV. Results are given that show clear evidence of a superionic behaviour close to the melting point, similar to the one found in alkaline earth fluorides and several halides. Furthermore, X-ray diffraction experiments on a high optical quality single crystal were performed. The cell parameter and the population parameter of Na⁺ were accurately determined (6.5373 Ã… and 0.988, respectively).