  In studies using piezoresponse force microscopy, we observe a non-zero
lateral piezoresponse at 180$^\circ$ domain walls in out-of-plane polarized,
c-axis-oriented tetragonal ferroelectric Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$
epitaxial thin films. We attribute these observations to a shear strain effect
linked to the sign change of the $d_{33}$ piezoelectric coefficient through the
domain wall, in agreement with theoretical predictions. We show that in
monoclinically distorted tetragonal BiFeO$_3$ 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.
