Thermodynamical Theory of Equisymmetric Ferroic-to-Ferroic Transitions

A thermodynamical theory is presented of the transition sequence [prototypic→ferroic→ferroic] where the two ferroic phases have the same symmetry. The so-called “two-sublattice model” is not adopted; instead, an order parameter in the ordinary sense is employed (it is denoted by Q ). In the free energy function, the eighth-degree term, which has conventionally been ignored, is given a due regard. It plays a vital role. The conditions for the equisymmetric ferroic-to-ferroic transition to occur are found. The transition temperature and transition pressure are determined. The temperature range of each ferroic phase under a constant pressure and the Q s vs T relation over the whole of that range are deduced, with the use of the mathematics of cubic equations.