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

For pt.I see ibid., vol.16, p.1353 (1983). The low-frequency (0-500 cm -1 ) Raman spectra of (CH 3 NH 3 ) 2 CdCl 4 single crystal and of its deuterated homologues (CD 3 NH 3 ) 2 CdCl 4 and (CH 3 ND 3 ) 2 CdCl 4 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 CdCl 6 octahedra sublattice. The peculiar selection rules observed for a low-lying overdamped mode, assigned to oscillations of the rigid CdCl 6 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.