LATTICE VIBRATION SPECTRA Part LXIV. Raman spectroscopic study of lithium hydrogensulfide LiSH: dynamic disorder and order-disorder phase transition

Article abstract of DOI:10.1016/0022-2860(91)87097-2

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 exhibits 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.