RAMAN-SCATTERING STUDY OF POTASSIUM CALCIUM-FLUORIDE KCAF3

Raman spectra of the perovskite crystal KCaF3 were recorded between 30 and 700 K and assigned in the monoclinic room temperature phase. Comp ared to the 14A(g) + 10B(g) Raman modes predicted by group theory calc ulations, only 9A(g) and 7B(g) modes were observes. Three soft modes, due to condensation of zone boundary modes of the R-M phonon branch of the cubic phase were observed and shown to display a Landau-type beha vior. The observation of a central component and the evolution with te mperature of characteristic parameters of hard Raman modes, close to t he transition, are reported and discussed in terms of a precursor diso rdered phase preceding the lower temperature phase.