Lattice dynamics and structural phase transitions in perovskite-type layer compounds. II. The analysis of the temperature-dependent Raman spectra of (CH3NH3)2ClCl4

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Abstract
For pt.I see ibid., vol.16, p.1353 (1983). The low-frequency (0-500 cm-1) Raman spectra of (CH3NH3)2CdCl4 single crystal and of its deuterated homologues (CD3NH3)2CdCl4 and (CH3ND3)2CdCl4 have been recorded at various temperatures covering the domains of existence of the disordered orthorhombic room-temperature (ORT) and tetragonal low-temperature (TLT) phases. An assignment of the observed Raman-active lattice vibrations is proposed; it is based upon the observed polarisation selections, the comparison of the spectra of the various isotopic derivatives and of the spectra previously analysed for the monoclinic low-temperature phase. It is concluded that in the ORT and TLT phase disorder involves not only the orientation of the methyl ammonium groups but also the CdCl6 octahedra sublattice. The peculiar selection rules observed for a low-lying overdamped mode, assigned to oscillations of the rigid CdCl6 octahedra, are interpreted using the hypothesis of existence of short-range correlations. A new qualitative approach for the transition mechanism in this system is tentatively given.