The theory of phase diagrams of thermodynamic systems with symmetry

Phase diagrams of the crystals undergoing phase transitions induced by irreducible representations with the L = 4 ¯ 3 m symmetry group are constructed within the phenomenological theory of the second-order phase transitions. Model thermodynamic potential of the eighth power of expansion in order parameter is investigated in detail. The equations of state of all phases induced by symmetry are received, and the general conditions of their thermodynamic stability are formulated. Some types of splitting of multicritical point in phase diagram are investigated by computer calculations. For the first time the possibility of existence of phase diagrams containing one or several triple points, three-phase and four-phase regions is shown. Examples of the crystals undergoing phase transitions with considered symmetry of the order parameter are given.