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Many studies have been devoted to investigate the cubic to tetragonal phase transition in RbCaF3 at 194 K. In order to describe the weak first order character of this phase transition, we have carried out calculations with a phenomenological Hamiltonian which contains, in addition to the expression proposed by SLONCZEWSKI and THOMAS, the six order terms of the order parameter. The results, obtained for the temperature dependence of the lattice parameters, the specific heat and the elastic constants, give a good qualitative description of the anomalies observed in the vicinity of the phase transition.

We report on the first inelastic neutron scattering investigations of the layer material KAIF4 which undergoes a martensitic phase transition. It is shown that the transition is preceeded by the softening of a flat phonon branch. The atomic displacements corresponding to these phonons are determined by a rigid ion model; a mechanism has been proposed to explain how the martensitic transition can be related to the phonon softening.

The tetrafluoroaluminates AA1F4 with layer structure undergo several structural phase transitions which can be related to soft modes condensation. We propose a rigid ion model to describe the lattice dynamics properties of these materials. The results of the model applied to RbA1F4 are presented. The calculated phonon spectrum reveals the presence of low frequency modes which are consistent with the sequence of transitions observed in this compound.

We report some preliminary inelastic neutron scattering experiments performed in the vicinity of the structural phase transitions in KCaF3. The analysis of the data displays the occurence of a central peak all along the R-M line of the cubic Brillouin zone.