Low-frequency raman spectra and phase transitions in perovskite-type layer compounds. (CH3NH3)2MnCl4 and (CH3NH3)CdCl4

The low-frequency (0 to 500 cm−1) Raman spectra are recorded at different temperatures between 40 and 413 K through the different structural modifications: P21/c ↔ P42/ncm ↔ Cmca ↔ ↔↔ I4/mmm. The results agree with previous NMR data and show that the three higher-temperature tetragonal and orthorhombic phases exhibit specific disorder with respect to the orientation of the methylammonium cation while the monoclinic low-temperature modification is ordered. A soft-mode behaviour is observed in the intermediate P42/ncm and Cmca phases, associated with the high-temperature second-order Cmca ↔ I4/mmm transition. A discussion of the unusual phase sequence in these materials is tentatively given, in view of previously proposed mechanisms for the phase transitions.