Using a serial bilayer model we show that heterogeneous ferroelectric systems may exhibit substantial Maxwell–Wagner piezoelectric relaxation. The relaxation has its origin in the coupling of the dielectric and piezoelectric properties. The model predicts both retardation (positive phase angle and counterclockwise charge–pressure hysteresis) and relaxation (negative phase angle and clockwise charge–pressure hysteresis) of the longitudinal direct piezoelectric coefficient. The theoretical results are confirmed experimentally in ferroelectric ceramic–ceramic bilayers and in a single-phase ferroelectric Aurivillius compound ceramic in which neighboring grains with strongly anisotropic properties may behave as basic Maxwell–Wagner units.

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