Phenomenological models of phase transitions for two interacting order parameters, one–component and
two-component, are constructed. The construction was carried out based on the number of control
parameters depending on external thermodynamic conditions by methods of the equivariant catastrophe
theory (singularity theory), taking into account the symmetry of the order parameters. The classification of
phenomenological models according to the number of control parameters is carried out. Phase diagrams of
the models are constructed and the theoretical temperature dependence of the heat capacity is calculated,
which shows a satisfactory qualitative correspondence with the experimental dependence in the Rb2KInF6
elpasolite crystal