Optical and X-ray evidence of structural phase transitions in mixed (Rb_1−xK_x)CaF₃ crystals

Abstract

Structural phase transitions have been found in the mixed fluoroperovskites crystals (Rb_1−xK_x)CaF₃ (x=0.05, 0.2, 0.35, 0.5, 0.88, 1.0) by optical birefringence and X-ray diffraction.

Profile analysis of powder diffraction lines was performed to determine the change of lattice parameters versus temperature in order to establish the crystal symmetry of each structural phase and to measure the temperatures of the transitions. From the splitting of the 400 cubic Bragg line and consideration of Glazer's classification of tilted octahedra in perovskites, the various types of distortions were found. Sequences of CaF₆ tilting octahedra occurring during the phase transitions are suggested and discussed for each concentration.

Key Words:

• Perovskites-type compounds
• X-ray and optical retardation
• structural phase transitions