Influence of the cationic substitution on the mechanism of structural phase transitions in RbCaF3 and KCaF3: study of a typical mixed crystal Rb0.68K0.32CaF3

A full investigation of structural instabilities in the mixed perovskite cr ystal Rb0.68K0.32CaF3 was performed by optical birefringence, X-ray diffrac tion and Raman scattering. A sequence for the phase transitions, according to the Glazer formalism, is proposed in agreement with that occurring in bo th the pure crystals RbCaF3 and KCaF3. Raman spectra are interpreted using group theory analysis and discussed in the framework of a classical Landau behaviour. With this work, the unusual phase transition mechanism observed for the perovskite KCaF3 appears accessible to experiment and is then studi ed in detail.