LASER RAMAN-STUDY OF THE LOW-TEMPERATURE PHASE-TRANSFORMATION IN [N(CH3)4]2CUCL4 SINGLE-CRYSTALS

The results of a detailed temperature-dependent Raman study of single crystals of [N(CH3)4]2CuCl4 are reported in different spectral regions. The observed splittings of the symmetric stretching modes of the N(CH3)4+ ion below 250 K and the CuCl4(2-) ion below 50 K into doublets have been explained as being due to the presence of ions on two energetically inequivalent positions within the lattice. From a temperature dependence of the half-widths, frequency separation, relative intensities of the two components of the C-N stretch and other thermosensitive spectral changes, we have discovered a new first order type phase transition in this system at approximately 50 K due to ordering of ions from one configuration to another. The reordering in phase IV up to 50 K takes place via continuous movement of cations from the disordered to ordered configuration, but below 50 K the ions freeze in the two inequivalent configurations. The dynamics of ordering of the ions at different temperatures is discussed in this communication.