192 K PHASE TRANSITION IN RbCaF3 : I LATTICE DYNAMICS.

The squared frequency of the R//2//5 soft mode is calculated as a function of the interatomic force constants of a rigid ion model. Analytical expressions of the parameters lambda //1, lambda //2 and lambda //3 characterizing the anisotropy of the dispersion surface near the R//2//5 mode are given. The calculated low frequency dispersion branches are compared with the available experimental data. All these results are discussed in terms of CaF//6 octahedron rotations.