Raman spectra and structural phase transitions in chain compounds (CH3)4NMnCl3 and (CH3)4NCdCl3

The Raman spectra (0 to 1000 cm−1) of the isostructural compounds (CH3)4NMnCl3 (TMMC), (CD3)4NMnCl3 (d-TMMC) and (CH3)4NCdCl3 (TMCC) are recorded at different temperatures (from 25 to 300 K) and analysed in view of the previously reported structural data and phases transitions. The order-disorder monoclinic-hexagonal phase transition occurring at 126 K in TMMC and d-TMMC is characterized in particular by the appearance, in the hexagonal disordered phase, of a diffuse scattering centered at ω = 0, due to the “rotatory” mode of the MCl3 chains coupled with a (CH3)4N+ ion (TMA) overdamped libration. In TMCC two transitions occur at 118 and 104 K. The mechanism for the monoclinic-hexagonal transformation at 118 K is probably related to a change in the TMA ion configuration. Furthermore, the observed Raman spectra give some evidence for the existence of a new disordered state of the TMA ions in this intermediate phase. The transition at 104 K is then related to a complete ordering of the TMA ions.