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.
The evolution of the internal and external modes in the Raman spectra of NaClO, has been studied in the neighbourhood of the transition temperature, at 581 K. Large changes in the frequencies of the external modes with temperature have been observed. These changes can be explained in terms of a model based on the reorientation of the perchlorate ions. INTRODUCIION A N b Y S I S OF THE NORMAL MODESMany authors have dealt with the vibrational spectra of perchlorates both in solution'" and as solids at room temperature and at low t e m p e r a t~r e .~,~ Studies of solid perchlorates have included both infrared ~pectra''~-' and Raman ~pectra.~'' In this paper, we report a study of the changes with temperature in the Raman spectra of anhydrous sodium perchlorate NaC104, from room temperature to the neighbourhood of the phase transition temperature T, = 308 "C. This is an order-disorder transition and is characterized by a disorder in the orientation of the ClOi ions. In the disordered hase, the structure is described by the space group Oh (Fm3m), if we do not consider the position of the oxygen atoms. Below T,, the structure is described by the space group 0:; (Cmcm). We shall show that this transition can be explained by the mean of a two-dimensional order parameter; and we shall give group theory arguments that satisfactorily explain the evolution of the phonon spectra.
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