On some features of the phase diagram of a ferroic with the cubic paraphase

The phase diagram of a ferroic with the paraphase of symmetry O _h has been studied for a three-dimensional order parameter. In the simplest model, which allows for the existence of a phase of the general position, i.e., the triclinic phase, the region of its existence has been investigated analytically. The invariance of the shape of this region is shown. The conditions providing the formation of a “swallowtail”-type feature in the region of existence of the monoclinic phase are considered. A convenient parametrization is proposed for the representation of the phase diagram, including all possible phases.