Abstract
A full investigation of structural instabilities in the mixed perovskite crystal Rb0.68K0.32CaF3 was performed by optical birefringence, X-ray diffraction and Raman scattering. A sequence for the phase transitions, according to the Glazer formalism, is proposed in agreement with that occurring in both 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 studied in detail.