Statistical Theory of Ferroelectric Phase Transitions of the Order-Disorder Type

As has been noted earlier, from the standpoint of the microscopic mechanism of a phase transition it is reasonable to consider separately a group of ferroelectric crystals with an order-disorder phase transition. In accordance with Chap. 1, we will use the term order-disorder ferroelectrics for crystals in which at least one of the sublattices consists of particles having two or more equilibrium positions. It may be said that the remaining (ordered) sublattices create, as it were, a “skeleton” of the crystal and the atoms that belong to these sublattices perform slight oscillations relative to fixed equilibrium positions. It is their configuration that creates potentials of two or more minima for sublattice atoms (see Fig. 7.2).