Structural phase transitions in (CH3)3NHCdCl3 studied by low-frequency Raman scattering

The low-frequency Raman spectra (0 to 300 cm−1) of (CH3)3NHCdCl3 single crystal are studied at different temperatures ranging from 90 to 423 K. Between 90 and 342 K, the observed lattice vibrations and polarization selections completely agree with the Pbnm (Z = 4) space-group previously reported. However, marked spectral changes occurring at 240 to 250 K are assigned to the occurrence of an order-disorder phase transition. This phase transition does not change the space-group Pbnm. Transitions at 342 and 374 K are clearly detected: the spectra obtained in these high temperature phases are consistent with the occurrence of complex hexagonal unit-cells, as determined from X-ray data by Chapuis et al. The mechanism for this phase sequence is believed to be related to the reorientational dynamics of the (CH3)3NH+ groups, coupled with “rotations” of the CdCl6 octahedra via NH … Cl hydrogen bond interactions.