  Results of detailed investigations of stability of phases in substances with
the small difference in the energies of the ferroelectric and the
antiferroelectric types of dipole ordering are presented. It is shown that
interaction of locally separated domains of the ferroelectric and
antiferroelectric phases stabilizes a nonuniform state of the samples. The
analysis of physical phenomena caused by the presence of domains of coexisting
ferroelectric and antiferroelectric phases in different dipole ordered
substances is presented. The consideration of peculiarities of behavior of
systems with nonuniform state of coexisting phases under the action of changing
external thermodynamic parameters (temperature, pressure, electric field, and
chemical composition) is given.
