STRUCTURAL INSTABILITIES IN DISORDERED PEROVSKITES RB1-XKXCAF3 STUDIED BY SYNCHROTRON-RADIATION POWDER DIFFRACTION - PROPOSITION FOR A PHASE-DIAGRAM

In the framework of general studies of structural phase transitions in disordered materials, the crystal structure of mixed perovskites Rb1- xKxCaF3 has been investigated by high resolution powder diffraction us ing synchrotron radiation, for three selected impurity concentrations x = 0.05, 0.20, 0.32. The symmetries and space groups have been fully determined according to Glazer's formalism and the structures refined using the Rietveld method. Special attention was paid to pre-transitio nal effects and phase coexistence. Thanks to this thorough examination , a full phase diagram, transition temperature versus concentration, i s suggested and interpreted from a model using the replica trick metho d and the mean field approximation. The validity of this model is disc ussed.