THE INFLUENCE OF SUBSTITUTIONAL DISORDER ON THE STRUCTURAL PHASE-TRANSITIONS IN RB(1-X)K(X)CAF3 (X LESS-THAN-OR-EQUAL-TO 0.2) MIXED PEROVSKITE CRYSTALS

The influence of ionic substitution on the phase transitions in mixed Rb1-xKxCaF3 fluoroperovskite crystals (x less-than-or-equal-to 0.2) is studied by EPR of the Gd3+-O2- pair substituted for a Ca2+-F- bond as a local probe. The behaviour of the EPR Spectra versus temperature re veals the existence of two successive first-order phase transitions as sociated with CaF6 octahedron rotation. Computer reconstructions of th e EPR spectra allow us to conclude that, in the low-temperature phase, the CaF6 octahedra are rotated by three equal angles around the fourf old cubic axes. Then, we were able to determine the variations of the total rotational angle phi defined by phi2 = phi[100]2 + phi[010]2 + p hi[001]2 = 3phi[001]2. It is observed to increase continuously for x = 0.05 whereas a discontinuity towards larger values occurs in the x = 0.2 compound. The phase diagram drawn from the EPR measurements shows that the transition temperatures are enhanced with increasing x and th e temperature range over which the tetragonal phase exists tends to be come smaller. These results lead to the conclusion that the occurrence of the phase transitions is related to the compactness of the unit ce ll and that the substitution enhances the instabilities at the M and R points of the Brillouin zone.