We report a Raman scattering investigation of CsPbCl 3 as a function of temperature in four different scattering configurations allowing us to obtain the various irreducible representations of the appropriate symmetry groups describing the substance in its successive phase transitions: cubic, tetragonal, orthorhombic and monoclinic or orthorhombic. We identify the correct 'parentage' of the observed modes with respect to infrared reflectivity data reported in the literature in accord with the Lyddane-Sachs-Teller relation, which was not fulfilled in previous assignments. We demonstrate the presence of strong correlation chains, which maintain the compound in a pseudo-cubic state as far as x-ray, infrared and Raman spectroscopy are concerned. The intensity of the related central peak was analysed on the basis of a Debye-type relaxation as a function of temperature; the results show perfect correspondence of its variations and the temperatures of the phase transitions. In addition, an order-disorder phase transition is evidenced at low temperature, 193 K (−80 • C).

Single crystals of 2-amino-5-nitropyridineL-(+)-tartratc (ANPT) have been studicd by Raman scattering and infrarcd abso tion at room temperature and at 10K. The crystal of monoclinic symmetry space group P21 (C,) undergoes no detectable structural phase transition between these two temperatures. The room temperature spectra were compared with those of parent compounds in order to propose a tentative assignment of the observed vibrational modes. Particular attention was paid to the hydrogen bond rnodcs.

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