Phenomenological theory of multicritical points splitting in the phase diagrams. The case of Landau’s potentials with ℒ = 3m

The detailed research of phase diagrams is carried out for Landau’s thermodynamic potential with ℒ = 3 m symmetry group. All possible types of phase diagrams for the sixth-order expansion of thermodynamic potential in terms of the order parameter components are received. Realization conditions for each type of phase diagrams are established, conditions of the multicritical point splitting are studied, and the equations for calculation of tricritical and triple points coordinates are obtained. Surfaces of order parameter components as functions of thermodynamic potential parameters are designed. Qualitative correspondence of phase diagrams theoretical calculations results and an experimental phase diagram is noted at the example of spinel structural type.