In the framework of general studies of structural phase transitions in disordered materials, the crystal structure of mixed perovskites has been investigated by high resolution powder diffraction using 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-transitional effects and phase coexistence. Thanks to this thorough examination, a full phase diagram, transition temperature versus concentration, is suggested and interpreted from a model using the replica trick method and the mean field approximation. The validity of this model is discussed.