The structural stability of the phenomenological models with the symmetry group L=3 m and their phase diagrams

We propose a method of investigating the structural stability of phenomenological models of the phase transition by means of the singularity theory (catastrophe theory) combined with the Poincare normal form theory. Also we find the structurally-stable thermodynamic potential models invariant under the order parameter symmetry group L = 3 m and with n = 1∼5 varying parameters, which are defined in the ranges of modules satisfying the conditions of global minimality and thermodynamic stability. And various phase diagrams of the potential models are constructed, and based on these, the theoretical T-x-P phase diagram for Ni1-xCuxCr2O4 solid solutions is constructed, which is in good agreement with the experimental data.