In Situ XRD Observation of Crystal Deformation of Piezoelectric (K,Na)NbO₃ Thin Films

(K,Na)NbO3 (KNN) thin films are promising lead-free piezoelectric materials for microelectromechanical systems (MEMS) devices. However, the origin of the strong piezoelectric properties of KNN thin films remains unclear because crystallographic deformation by piezoelectric effects is not clear in detail. We used synchrotron X-ray diffraction (XRD) to explore the origin of the piezoelectricity of polycrystalline (K0.45Na0.55)NbO3 (KNN) thin films, which led to the observation of large crystal deformation originating from the piezoelectric effects. The peak shifts of the XRD patterns indicated changes in both the out-of-plane and in-plane lattice parameters of KNN. In addition, an electric field-induced phase transition under an applied electric field was observed. The microscopic piezoelectric coefficients (e31,f) were estimated from the in situ XRD results and subsequently compared with the macroscopic piezoelectric coefficients estimated from the converse piezoelectric effect by the cantilever method. The macroscopic |e31,f| coefficients based on the converse piezoelectric effect were in the range of 6.3–11.1 C/m2, whereas the microscopic |e31,f| values based on the in situ XRD results were in the range of 1.2–1.5 C/m2. However, the macroscopic piezoelectric coefficients from the direct piezoelectric effect were 1.6–2.0 C/m2, which were similar to those obtained from the in situ XRD results. The results suggest that the large macroscopic piezoelectric properties associated with the converse piezoelectric effect arise from the observed electric field-induced phase transition. This study demonstrates the main factors associated with the macroscopic piezoelectric properties in lead-free KNN thin films.