LATTICE VIBRATION-SPECTRA .64. RAMAN-SPECTROSCOPIC STUDY OF LITHIUM HYDROGENSULFIDE LISH - DYNAMIC DISORDER AND ORDER-DISORDER PHASE-TRANSITION

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 tranition 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.