96-17E

In the context of description of structural phase transition, the anharmonic potential effecting on the ion in the crystal lattice is selected in the form that, in addition to quadric, includes cubic anharmonicity, thus possessing the most general non-symmetrical form; the interaction among ions in different cells is accounted in the mean field approximation. The dependencies of the order parameter, free energy and dielectric susceptibility on the externally applied field and the order parameter and free energy on the temperature are derived for various model parameters. The phase diagram (external field, temperature) is derived and the influence of cubic anharmonicity on the form of the phase diagram and location of the critical point is studied. The first order phase transition, occuring as the external field varies as well as the temperature, is studied.