Dynamical aspects and thermodynamic theory of the α-β phase transition in RbCrCl₃and CsCrCl₃crystals

A group theoretical analysis of the alpha - beta phase transition in RbCrCl 3 and CsCrCl 4 crystals is performed. The soft model transforms according to the E 1g irreducible representation belonging to the centre of the hexagonal Brillouin zone ( Gamma point). From the knowledge of the symmetry of the order parameter a Landau free-energy expansion is constructed, and expected anomalies in material constants around the transition temperature are briefly discussed.