Pairing High Piezoelectric Coefficients, d₃₃, with High Curie Temperature (T_C) in Lead-Free (K,Na)NbO₃

The largest piezoelectric properties, d₃₃ = 416 pC/N and 490 pC/N, in K_xNa_1-xNbO₃ ceramics have been reported for compositions close to polymorphic phase transition (PPT); however, they also have Curie temperatures, T_C, of around 217-304 °C, considerably lower than those of undoped KNN ceramics (420 °C). High d₃₃ along with high T_C remains the ideal choice for applications but, unfortunately, not attained up to now. Here, we show that using KNN single crystals as seeds for template grain growth (TGG) of KNN ceramics enables dramatic improvements in the electromechanical properties while maintaining a high T_C. The (001)-oriented (K_0.5Na_0.5)_0.98Li_0.02NbO₃ ceramics engineered by TGG using (K_0.5Na_0.5)NbO₃ crystals as templates exhibit a high d₃₃ of 280 pC/N while maintaining the high T_C of 430 °C. Enhanced piezoelectricity is attributed to long-range ordered ferroelectric domain patterns consisting of 90° and 180° domains, similar to single crystals. It is the first time that pairing high d₃₃ and high T_C in KNN, keeping a high PPT temperature, is achieved. This study is an unequivocal proof that it is possible to maximize d₃₃, keeping a high T_C in KNN without resorting to heavily doped compositions. This work opens the door to high-performance, rare-earth free, compositionally simple lead-free and low-cost electromechanical compounds, which can largely expand lead-free piezoelectrics applications.