  This paper contains results of investigations of inhomogeneous states caused
by the coexistence of ferroelectric and antiferroelectric phases in
lead-zirconate-titanate based solid solutions. The domains of ferroelectric and
antiferroelectric phases with sizes of the order of 20 to 30 nm coexist in the
bulk of the samples due to a small difference in the free energies of the
phases. The coherent character of interphase boundaries leads to the
concentration of elastic stresses along these boundaries. Elastic stresses
cause the local decomposition of the solid solution due to the circumstance
that equivalent positions of the crystal lattice are occupied by ions with
different sizes. Larger ions are driven out into domains with larger crystal
cell parameters and smaller ions are pushed into domains with smaller
parameters of the crystal cell. The sizes of segregates formed in this way are
of the order 8 to 15 nm.
