TEMPERATURE AND HIGH PRESSURE RAMAN STUDY OF M3P7 (M equals Li, Na, K, Rb, Cs) COMPOUNDS.

We report the results of a study of M//3P//7 (M equals Li, Na, K, Rb, Cs) compounds by Raman scattering at various temperatures and pressures in the solid state as well as in the solution. The compounds M//3P//7 (M equals Na, K, Rb, Cs) show a first order phase transition from a crystalline to plastic crystalline state. Above the transition temperature T//c the low frequency external Raman lines vanish completely for all compounds except Li//3P//7, which is characteristic for a transition from a crystalline to a plastic crystalline phase. In the case of Li//3P//7 the external Raman modes are still present above T//c, suggesting that the phase transition in Li//3P//7 involves a valence fluctuation within the polyanion P//7**3** minus .