The theory of phase diagrams of thermodynamic systems with symmetry 3m

Within the framework of the phenomenological theory of the second-order phase transitions for the crystals undergoing structural transformations, induced by irreducible representations with symmetry group L=3mL=3m, all possible types of phase diagrams are constructed. Conditions of realization of each type of phase diagrams are found in the approach of the thermodynamic potential of the sixth power of expansion on order parameter components. The equations for calculation of tricritical and triple point coordinates are received. In the case of spinel-type structure, qualitative agreement between the results of theoretical calculations of phase diagrams and experimental phase diagrams is observed.

► We analyze Landau model thermodynamic potential with symmetry 3m.
► We describe phase diagrams with a multicritical point and diagrams with splitting of a multicritical point.
► We describe three possible types of multicritical point splitting.
► The equations for calculation of tricritical and triple point coordinates are received.