Shear effects in lateral piezoresponse force microscopy at 180° ferroelectric domain walls

In studies using piezoresponse force microscopy, we observe a nonzero lateral piezoresponse at 180° domain walls in out-of-plane polarized, c-axis-oriented tetragonal ferroelectric Pb(Zr_0.2Ti_0.8)O₃ epitaxial thin films. We attribute these observations to a shear strain effect linked to the sign change of the d₃₃ piezoelectric coefficient through the domain wall, in agreement with theoretical predictions. We show that in monoclinically distorted tetragonal BiFeO₃ films, this effect is superimposed on the lateral piezoresponse due to actual in-plane polarization and has to be taken into account in order to correctly interpret the ferroelectric domain configuration.