EVIDENCE OF DISORDER IN THE PEROVSKITE CRYSTALS RBCAF3 AND RB0.65K0.35CAF3 - INVESTIGATION OF PHONON ACOUSTIC MODES BY INELASTIC NEUTRON-SCATTERING

From preliminary inelastic neutron scattering experiments performed in the vicinity of the cubic to tetragonal structural phase transitions in pure RbCaF3 (T-c = 193 K) and in mixed Rb0.65K0.35CaF3 (T-c = 291 K ) fluoroperovkite crystals, we report the evolution of energy line wid th of the LA and TA acoustic phonon branches along the TX(0,0,xi) dire ction when approaching T-c. The significative increasing of the line w idth observed for the two concentrations can be interpreted in the fra mework of a model that predicts the existence of an intermediate disor dered phase between tetragonal and cubic symmetry.