High electrostrain in a lead-free piezoceramic from a chemopiezoelectric effect

Piezoelectric materials are indispensable in electromechanical actuators, which require a large electrostrain with a fast and precise response. By designing a chemopiezoelectric effect, we developed an approach to achieve a high electrostrain of 1.9% under −3 kV mm−1, at 1 Hz, corresponding to an effective piezoelectric coefficient of >6,300 pm V−1 at room temperature in lead-free potassium sodium niobate piezoceramics. This electrostrain has satisfactory fatigue resistance and thermal stability, and low hysteresis, far outperforming existing lead-based and lead-free perovskite counterparts. From tracer diffusion, atomic optical emission spectrometry experiments, combined with machine-learning molecular dynamics and phase-field simulations, we attribute the high electrostrain to short-range hopping of oxygen vacancies near ceramic surfaces under an alternating electric field, which is supported by strain levels reaching 3.0% under the same applied field when the sample was annealed at a low oxygen partial pressure. These findings provide an additional degree of freedom for designing materials on the basis of defect engineering, which will favour not only the electrostrain of piezoelectrics but also the functional properties of a broader range of oxide-based materials. The authors demonstrate a chemopiezoelectric effect in which the displacive migration of oxygen vacancies driven by an electric field induces a large strain in the surface layer of thin (K,Na)NbO3 ceramics. They achieve an electrostrain of 1.9% under a field of −3 kV mm−1, with thermal stability up to 200 °C.