On the structures of the low-temperature phases of (CH₃)₄NMnCl₃and (CH₃)₄NCdCl₃: a Raman scattering and group theoretical study

The Raman spectra of the isomorphous compounds (CH 3 ) 4 NMnCl 3 , (CD 3 ) 4 NMnCl 3 and (CH 3 ) 4 NCdCl 3 in the regions of the lattice vibrations and of the internal modes of the tetramethylammonium skeleton are reinvestigated at temperatures ranging from 300 to 25K. On the basis of the hypothesis that group-to-subgroup relations exist between the high- and low-temperature phases of these compounds, a group theoretical analysis of possible phase transitions has been made. A Landau-type treatment of these phase transitions as they are expected to occur in TMMC and TMCC supports these conditions and in addition suggest the possibility of the existence of incommensurate phases in these systems.