Phenomenological model of a sequence of phase transitions in $(\mathrm{N}R_4)_2MeX_4$ crystals

A phenomenological model of a sequence of phase transitions in [N(C$_2$H$_5$)$_4$]$_2$CuCl$_4$ crystals has been proposed. The model is constructed by methods of singularities of differentiable maps (the catastrophe theory). In this model, low-temperatures anomalies of thermal properties (entropy and heat capacity) are interpreted as isomorphic phase transitions or as a supercritical behavior on a thermodynamic path in the phase diagram near liquid-vapor end points.