Polarized Raman spectra of oriented single crystals of KCIO, performed over a large temperature range (300-800 K) have been used to investigate the mechanism of the 583 K phase transition. It is shown that the transition can be described using an order parameter and a mechanism based on the reorientation of the Clodions is proposed. A Raman study of the plastic phase (above 583 K), using the shape of the bands associated with the internal modes, leads to a measure of the orientational density probability and the orientational self-correlation functions of the perchlorate ions.

Raman spectra of RbHSO, have been recorded in the temperature range from 6 K to the melting point, 480 K. Analysis of the spectra show that the second order phase transition, at 264 K, is of the order-disorder type. NO soft mode was found. The evolution of the v(0-H) mode frequency with temperature, is consistent with a change of orientation of the hydrogen bonds connecting the disordered SO,-O(H) groups. The spectra are also consistent with the existence of a prototype phase which hs3 been assumed to explain the pseudo-orthorhombic symmetry of the paraelectric and ferroelectric phases.

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Polarized Raman spectra of single crystals of NaH2PO4.2H20 and NaD2P04.2D20 have been measured as a function of temperature in the wavenumber shift range 10-3600 cm-'. Changes of phonon frequencies due to directional dispersion are found. All the observed bands have been assigned using group theory. The Raman spectra are fully consistent with the presence of strong acidic hydrogen bonds and the existence of a weakly bound water molecule.

Polarized Raman spectra of oriented single crystals of (CH,CH,NH,),SnCl, measured over the temperature range 15-300 K have been used to investigate the mechanism of the structural transition at 125 K. A detailed study of some of the external modes and of the internal modes of both the SnClk and CH3CH2NH: ions in the high-and low-temperature phases has enabled the structure of the disordered high-temperature phase to be confirmed and suggestions for the structure of the low-temperature phase to be proposed.

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The Raman spectra of the high-temperature (a) phase of crystalline malonic acid, CH2(COOH),, and its deuteriated derivative, CD2(COOD)2, have been investigated at 370 K in the wavenumber shift range 0-4000 cm-'. An assignment of the internal and external vibrations is given. Comparison of the infrared and Raman spectra of the a and p phases shows that the high-temperature phase (a) consists of equivalent centrosymmetric dimer rings and that the corresponding space group is G h -C 2 / c with molecules occupying C, sites. The temperature dependence of various Raman lines, in particular those due to lattice modes, indicates that the phase transition is of first order and that during the transition, reorientation of the molecules about the c axis is strongly coupled with the low-frequency torsional (yOH --0) and librational modes.

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