Raman scattering study of the non-ferroic phase transition of rbalf4.

The P4/mmm – P4/mbm structural phase transition of RbAlF4 at 553 K is studied in the framework of the Landau theory with a free energy involving two coupled order parameters. The model deduced from the anisotropy of the phonon dispersion curves relative to the soft mode makes it possible to explain the first-order character of the transition and to predict the existence of a new phase with P4/mmm symmetry. A Raman scattering study of the temperature behaviour of the soft mode is in agreement with such prediction.