  The contribution Delta_epsilon of extremely small motions of domain walls to
small-signal permittivity of a multidomain ferroelectric sample has been a
research issue for many years. In ferroelastic ferroelectrics such motions
contribute also to their piezoelectric (by Delta_d) and elastic (by Delta_s)
properties. Data about their simultaneous existence are scarce but those
available point to mutual proportionality of Delta_epsilon, Delta_d and
Delta_s, as expected. To understand the magnitude of extrinsic contributions,
the origin of the restoring force acting on domain walls must be understood. In
the present contribution the theory has been developed based on the model of a
plate-like sample in which the ferroelectric- ferroelastic bulk is provided
with a nonferroic surface layer. Motion of domain walls in the bulk results in
a change of electric and elastic energy both in the bulk and in the layer,
which provides the source of restoring force. This makes it possible to
determine all mentioned extrinsic contributions. We discuss the applicability
of the model to available data for single crystals and also for ceramic grains.
