  This work presents a continuation of our last paper, concerning the theory of
the response of an antiparallel domain structure in a plate-like electroded
sample to external electric field. The theory is based on the exact formula for
free energy of the system, formed of a central ferroelectric part, isolated
from electrodes (with a defined potential difference) by a surface layers. Our
calculations are applicable also to thin films. It is usual to use the term
`extrinsic' for the contribution of domain walls displacement to macroscopic
properties of a sample. In our last paper we discussed the extrinsic
contribution to permittivity. In this work we concentrate on extrinsic
contribution to piezoelectric coefficients in ferroelectrics which are
simultaneously ferroelastics. As an example, we calculate the extrinsic
contribution to d_36 piezoelectric coefficient of RbH2PO4, that was recently
measured in a wide range of temperature below Curie point.
