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.

We report evidence for the existence of a new incommensurate phase in a crystal of ammonium hydrogen oxalate hemihydrate. This phase is remarkable in two aspects: it exists only above a critical pressure P,, and the incommensurate wave vector, which is parallel to the vector e* of the reciprocal lattice, has the largest variation ever reported, varying continuously from 0.147c* at 4.3 kbar to -0.25c* at the maximum pressure (8 kbar) used to date. This letter reports results of a collaborative research, performed by complementary techniques, which has led to the discovery of an incommensurate phase in an insulating material, C204HNH4, BHzO, with two very unusual features:

C2O4HNH4, 1/2 H2O undergoes, at Tc = 145 K, a second order ferroelastic transition. The present Raman study allows us to identify the soft degree of freedom responsible for the transition as the ordering of one family of NH+4 ions. This ordering process appears as a central peak which displays a critical narrowing in the vicinity of Tc. Nevertheless, its linewidth is minimum not at Tc but below Tc . The combined analysis of its intensity and linewidth shows that this unusual behaviour is due to the evolution of the individual ionic relaxation time. The latter is strongly modified, below Tc, by the inequivalence between the two potential wells, as well as by the change of the potential barrier. This model gives quantitative agreement with our results

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.