Frozen-in trans and gauche conformational disorder of symmetrical difluorotetra-chloroethane (DFTCE) produces the tendency of this plastic crystal to form an orientational glassy state on cooling below Tg=86 K. To throw light upon the freezing mechanism in DFTCE neutron diffraction and Brillouin scattering as well as infrared and dielectric measurements are employed. The overall picture of the results obtained can be characterized in terms of a classical glass-transition process with a relaxation frequency obeying the Vogel-Fulcher law. However, a more sophisticated picture of the freezing process is indicated by a quantitative analysis of the new Brillouin data, e.g. the temperature dependence of the sound velocity. The behaviour could be understood on the basis of a mode-coupling theory.

The temperature-frequency behaviour of the permimivily d(T, f) and m[S(T. f)] (20-300 K, 120 Hz-9 MHz), EPR, optical absorption and luminescence of KTaO3, nominally pure and weakly doped (0.1 ~1 % Li: 0.01 wt% Cr; 3 wt% Cu; I wi% Fe; 0.1 wt% Li+O.l wt% CI and 0.1 wt% L i t 0 . l wt% Crt0.02 wt% Cu) were studied. Below 70 K, in all doped samples, dielecuic dispersion CCCUIS, with m[S(T)l maxima positions obeying rhc universal Arhenius law with activation energy A = 1000 * 50 K. This is associared with local ordering in the dipoledipole Clusters containing reorienting defect pairs of various kinds, with correlations due to interaction via t h e m soft mode.

Polarized Raman spectra of Cs2CdBr4 single crystals were measured in the frequency range 0-350 cm-' at 100-290 K. The spectra are analysed within the framework of the Landau theory of stmctural phase transitions; the analysis indicates that the transition to the triclinic phase with P i symmetry is accompanied by unit-cell volume multiplication.

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Polarized Raman spectra of Cs 5 H 3 (SO 4 ) 4 • H 2 O (PCHS) single crystals were measured in the frequency range 0-4000 cm −1 at 20-300 K, i.e. both below and above the glass transition temperature (T g ). The evolution of the spectra with decreasing temperature indicates that the orientational disorder of SO 2− 4 ions and H 2 O molecules changes at T g from the dynamical to a static one. A scenario for the glass transition and a model of the resulting glass state is suggested, in which the water molecules (H bonds) play the role of binding constituents of the isolated sulphate complexes H 3 (SO 4 ) 5− 4 .

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Polarized Raman and infrared specha of LiZGe70LS single crystals were measured in the frequency m g e 5-1000 cm-' at mom temperature. The spectra are analysed within the framework of the Landau thwry of Seucnual phase bnsitions from a (hypothetical) fully disordered deficient structure (aristotype). The mechanism of the phase transition is discussed in terms of ordering and shifts of ions.