Extrinsic contribution to piezoelectric properties of RbH2PO4 crystals in the ferroelectric phase

We have measured permittivity epsilon(3), elastic compliance s(66)(E) and piezoelectric coefficient dss of a single crystal of RbH2PO4 in a wide temperature region above and below the transition temperature T-tr. All these quantities are shown to be anomalously large within the interval of about 35 K below T-tr, due to the presence of domain walls. In particular, the value of d(36) exceeds 3000 pC/N in this interval. The model of pinned domain Rails oscillating around their equilibrium positions predicts the relation Delta epsilon(3)(sigma)/Delta d(36) = Delta d(36)/Delta s(66)(E) for the extrinsic contributions which is well satisfied bs the obtained data.