Phase transitions in tetrafluoroaluminates RbAlF4and KAlF4: brillouin scattering and ultrasonic propagation

Abstract

Abstract RbAlF4 and KAIF4 belong to the family MAIF4 (M = K, Rb, Tl) of layered tetrafluoraluminates undergoing several phase transitions starting from a real or virtual high-temperature tetragonal aristotype structure (phase I-P4/mmm, Z = 1) characterized by a two-dimensional arrangement of AlF6 octahedra in sheets separated by K+ or Rb+ cations. The main feature of both compounds is the dominant role of the rotations of the AlF6 octahedra related to softenings of normal modes which are involved in the mechanisms of the successively encountered structural transformations (Bulou and Nouet, 1982; Bulou, 1985). These transformations can be interpreted in the framework of a Landau model, taking into account the pertinent order parameters associated with the soft modes at appropriate values of the wave-vector and including in addition their coupling with the strains: this leads to predictions of the elastic behaviour in the vicinity of the phase transitions studied by measuring the temperature dependence of the stiffness constants determined from 150 K to 700 K using Brillouin light scattering and from 250 K to 350 K using ultrasonic propagation experiments.