Composition dependence of local piezoelectric nonlinearity in (0.3)Pb(Ni_0.33Nb_0.67)O₃-(0.7)Pb(Zr _xTi_1-x)O₃ films

The composition dependence of dielectric and piezoelectric nonlinearities were characterized in epitaxially grown (0.3)Pb(Ni_0.33Nb _0.67)O₃-(0.7)Pb(Zr_xTi_1-x)O ₃ thin films deposited on SrTiO₃. Tetragonal, morphotropic phase boundary (MPB) and rhombohedral films were prepared by changing the Zr/Ti ratio. The relative dielectric permittivity ε_r and the converse piezoelectric coefficient d_33,f were found to follow the Rayleigh law. The local piezoelectric nonlinearity map showed the formation of micron-sized clusters of higher nonlinear activity for the MPB and rhombohedral compositions. The ratios of the irreversible to the reversible Rayleigh constants α_ε/ε_init and the spatially averaged α_d/d_33,init ratio were larger for the rhombohedral and MPB compositions compared to the tetragonal composition. The larger dielectric and piezoelectric nonlinearities observed for the rhombohedral sample are interpreted in terms of a higher domain wall mobility due to a smaller ferroelectric distortion and superior crystal quality.