OPTICAL AND X-RAY EVIDENCE OF STRUCTURAL PHASE-TRANSITIONS IN MIXED (RB1-XKX)CAF3 CRYSTALS

Structural phase transitions have been found in the mixed fluoroperovs kites crystals Rb1-xKxCaF3 (x=0.05, 0.2, 0.35, 0.5, 0.88, 1.0) by opti cal birefringence and X-ray diffraction. Profile analysis of powder di ffraction lines was performed to determine the change of lattice param eters versus temperature in order to establish the crystal symmetry of each structural phase and to measure the temperatures of the transiti ons. From the splitting of the 400 cubic Bragg line and consideration of Glazer's classification of tilted octahedra in perovskites, the var ious types of distortions were found. Sequences of CaF6 tilting octahe dra occurring during the phase transitions are suggested and discussed for each concentration.