  Interrelations of domain wall (extrinsic) contributions to dielectric,
piezoelectric and mechanical properties of ferroic samples are a hot subject of
both theoretical and experimental research. Recently,we have derived
theoretical formulas for such contributions, using model of composite layer
sample: central single-crystal ferroelectric-ferroelastic layer, isolated from
electrodes by passive layers. Here we present more general results, discussing
the dependence of the contributions on geometric and material parameters of the
composite and including in a special case intrinsic piezoelectricity. We also
discuss the above-mentioned interrelation and compare our results with
measurements of all the contributions on the same RDP sample in a wide
temperature interval under the phase transition. It seems that both calculated
and observed results remind of the so called Pippard-Janovec thermodynamic
relations.
