Lattice vibration spectra

Abstract

Raman spectra (4000—50 cm −1 ) of lithium hydrogensulfide are recorded in the range from 70 to 300 K. The temperature dependence of frequencies and halfwidths of the SH − stretching and librational modes yields clear evidence for a disorder—order phase transition at 222 K and thermally activated dynamic disorder of the SH − ions in the room temperature polymorph. Down to 70 K the increasing splitting of the in-plane libration can be interpreted by a Landau-type order parameter. This splitting reflects increasing dynamic interactions (factor group splitting) of adjacent (in[100]) SH − ions due to the ordering process. The halfwidths of the stretching and out-of-plane librational modes exhibit an Arrhenius-type behaviour. The activation energies derived are 1.9 kJ mol −1 for both vibrations. The activation temperature (230 K) is in good agreement with that of the change of the specific heat at 228 K. The order mechanism and a plausible structure of the ordered phase below 222 K are discussed.