Orientation dependent intrinsic and extrinsic contributions to the piezoelectric response in lead-free (Na_0.5K_0.5)NbO₃ based films

The intrinsic and extrinsic contributions to the piezoelectric response are deeply dependent on the crystallographic orientation in the thin films. Herein, (100), (110) and (111)-oriented (Na_0.5K_0.5)NbO₃ (KNN) based lead-free piezoelectric films are synthesized by a sol-gel method. The dielectric, ferroelectric and piezoelectric properties of the KNN-based films at the rhombohedral-orthorhombic (R-O) phase boundary are investigated with the focus on the influence of the film orientation. The maximum piezoelectric response is obtained in the (100)-oriented film, resulting from the extrinsic contribution stimulated by the external electric field, although the (110) and (111)-oriented films have the higher dielectric constant and remnant polarization. There are more varied ways of domain switching and domain configuration under the external electric field in (100)-oriented film, leading to the higher extrinsic contribution to the piezoelectric response. The study of the orientation effect on the piezoelectric response can shed light on the exploration of optimal piezoelectricity by tailoring the preferred orientation in KNN-based lead-free films.