Phenomenological Models of Ferroelastics with a Fully Symmetric Order Parameter: Classification by Methods of Equivariant Catastrophe Theory

Using the methods of the catastrophe theory, which take into account the symmetry of the thermodynamic system, phenomenological models of phase transitions in proper and pseudoproper ferroelastics have been constructed for interacting order parameters, one of which is fully symmetric, invariant with respect to all symmetry transformations of the crystal. A classification of models based on the number of control parameters, which depend on external thermodynamic conditions, has been carried out. A phase diagram of one of the models has been constructed, and the theoretical temperature dependence of the heat capacity has been calculated. The comparison of theoretical and experimental data showed satisfactory qualitative agreement.