  The Landau theory of 180 degrees domain walls in BaTiO3 type ferroelectric
particles is presented. Results of exact description of domain walls in bulk
enabled us to formulate variational approach to theory of domain walls in
corresponding small particles. The depolarization field effects and the
space-charge layers are taken into account in the samples of the cube form. It
was found that at low temperatures well known hyperbolic tangent wall profile
is a good approximation for description of domain walls. Near the transition
temperature it is more appropriate to speak about two walls separating
ferroelectric-paraelectric-ferroelectric domains correspondingly as a result of
splitting of a single ferroelectric wall in small BaTiO3 particles. Domain wall
energy density, average interwall distance and change of the dielectric
response of thick walls in small ferroelectric particles in microcomposites is
found. Our results describe observed dependencies better than those theories
which exist up to date. In temperature region near transition from the
ferroelectric to paraelectric phase in micrcomposites our new quantitative
results for response, for Curie-like transition temperature and other
properties may be verified.
