  Physical effects caused by the presence of interfaces between the domains of
coexisting ferroelectric and antiferroelectric phases in solid solutions with
small difference in free energies of the ferroelectric and antiferroelectric
states are discussed in this review. There exist a number of solid solutions in
which the two-phase state of domains of the coexisting phases may occur under
certain conditions. We present here results of investigations of some effects
directly caused by the presence of the interfaces between the domains of the
coexisting ferroelectric and antiferroelectric phases. The phenomenological
model describing the inhomogeneous state of coexisting domains of the
ferroelectric and antiferroelectric phases is presented. Experimental studies
of such inhomogeneous state are discussed using two systems of the
PbZr1-yTiyO3-based solid solutions. Detailed discussion of the process of local
decomposition of solid solutions in the vicinity of
ferroelectric-antiferroelectric interphase boundaries and formation of the
mesoscopic system of segregates in the vicinity of these interphase boundaries
and corresponding experimental results are presented. Results on influence of
the system of segregates on dielectric and piezoelectric properties, and also
on a dielectric relaxation are presented and discussed. The effects caused by
the application of a DC electric field are considered and corresponding
experimental results are presented. The experimental results demonstrating the
possibility of control of piezoelectric parameters by an external electric
field in materials with the antiferroelectric to ferroelectric phase transition
via the intermediate state of the coexisting domains of these phases are given.
