Peculiarities of thermodynamics of phase transitions in a system with two ordering channels

A Landau model potential taking into account the presence of interaction of two ordering channels is proposed. This potential corresponds to a phase diagram which is a common fragment of phase diagrams for many experimentally studied systems. It is shown that the existence of the second channel and the interaction of order parameters determining the ordering in the two channels are necessary conditions for a radical change in the thermodynamic parameters of a phase transition, such as the sign of curvature of the equilibrium transition line and of the lability boundaries of the ordered and disordered phases in the main order parameter of the phases, and the sign reversal of the thermal effect (which is single on the phase equilibrium curve and double on the lability boundaries). Some of these peculiarities are observed in the experimental phase diagrams, while others are predicted and can be detected in experimentally refined phase diagrams.