The phenomenological theory of the collapse tetracritical point of the phase diagram

In the framework of the phenomenological theory of phase transitions for the thermodynamic potential invariant under a group of transformations C$_{3V}$ (3m), the results of the analysis of the collapse of the multicritical point. The conditions of the decay of multicritical point are obtained, possible types of “derivative” phase diagrams are listed. As an illustration, we present some calculated phase diagrams. These results are consistent with experimental data.