STRUCTURAL PHASE-TRANSITIONS IN [(CH3)4N]CDCL3 STUDIED BY THE EPR OF THE [N(CH3)3]+ RADICALS

The hexagonal P6(3)/M to Monoclinic P2(1)/m structural phase transitio n in [(CH3)4N]CdCl3 has been investigated by the electron paramagnetic resonance (EPR) of the [N(CH3)3]+ radicals produced by gamma-irradiat ion. Above the transition temperature, this radical species exhibits a xial symmetry, with the hexagonal c-axis as the symmetry axis. Below t he phase transition, the magnetic symmetry departs significantly from axial symmetry. The apparent EPR proper rotation group in the monoclin ic phase is the hexagonal C6, the same as in the hexagonal phase. This indicates the presence of merohedral twinning below the phase transit ion. The hexagonal c-axis becomes the unique twofold axis of the monoc linic phase, and there exist three types of twin-domains related by ro tations of +/- 120-degrees (or +/- 60-degrees) about the c-axis.