Classification of Phenomenological Models of Phase Transitions with Three-Component Order Parameters by Methods of Catastrophe Theory: L=Td ((4)over-bar3m)

Using the equivariant catastrophe theory, we classify phenomenological models of phase transitions with a three-component order parameter and with a number of control parameters from one to four. The analysis of phase diagrams of the obtained models shows that the description of all low-symmetry phases requires fewer terms of the power-series expansion than that required by the model constructed using the traditional method taking all terms up to the 2nth power into account (n > 1). The theoretical temperature dependence of the heat capacity is compared with the experimental data in the GaV4S8 compound.