Micro-Raman spectra of langbeinite-type K2Mn2(SO4)(3) and (NH4)(2)Cd-2(SO4)(3), near the phase transition temperature

Raman spectra of the langbeinite-type crystals K2Mn2(SO4)(3) and (NH4)(2)Cd -2(SO4)(3) have been observed by using a micro-Raman scattering technique w ith a special attention to internal modes Of SO4 molecules. The observed pe aks are well assigned to the internal modes Of SO4 in both the high- and lo w-temperature phases. At the phase transition temperature, new internal mod es appear owing to the inequivalence of the molecular sites in the crystal. In addition, micro-Raman spectra have been observed at several positions o n the surface of a single crystal. Micro-Raman spectra just at the transiti on temperature show either the high-temperature structure or the low-temper ature structure as a function of the observed position. It is concluded tha t the sample single-crystal consists of the local regions which have differ ent transition temperatures. This result indicates that not the intermediat e phase but the coexistence of two phases appears in these langbeinite-type crystals.