Characteristics of giant piezoelectricity around the rhombohedral-tetragonal phase boundary in (K,Na)NbO₃-based ceramics with different additives

In this work, we confirmed the electric-induced transition (EPT) in (K,Na)NbO₃-based ceramics through experiment and theory. Through in situ X-ray diffraction measurements, electric field-induced phases (EPs) could be observed in the ceramics. To explain the appearance of EPs, a new function λ(E_ex) was introduced into the six order Devonshire theory when the external electric field was applied. Further studies indicate that EPT had two possible forms, i.e., tetragonal-electric induced phase transition (T-EP) and rhombohedral-electric induced phase transition (R-EP), and T-EP plays a more positive role than R-EP in terms of the piezoelectric response. Also, a giant piezoelectricity (d₃₃ = 435-490 pC N^-1), a high Curie temperature (T_C = 205-234 °C), and a converse piezoelectric coefficient (d∗33 = 500-890 pm V^-1) can be achieved by choosing optimum metal oxides as well as their content. We believe that such work could help to further study the physical mechanisms of giant piezoelectricity in potassium-sodium niobate.