Phenomenological description of the first order improper ferroelastic phase transition in RbCaF 3 at 194 K.

Abstract
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.