  Ferroelectric soft mode properties in ceramics and thin films are reviewed
and compared with single crystals. In general, the permittivity in ceramics and
thin films is markedly reduced compared to the single crystals mainly because
of low-permittivity grain boundaries - dead layers - (sometimes also
nano-cracks) present in the samples. In SrTiO3 and BaTiO3, this dielectric
reduction is fully explained by stiffening of the soft mode frequency. In most
other ferroelectrics, the soft mode accounts only partly for such a
permittivity reduction, since the transition is only partially driven by the
phonon mode softening. In these cases, additional relaxational dispersion below
the optical phonon range (central mode) accounts for the main part of the
dielectric anomaly near Tc. Influence of nano-cracks on reduced permittivity
near Tc and related stiffening of the central mode is manifested in the
antiferroelectric PbZrO3 ceramics.
