Achieving excellent piezoelectric properties with high Curie temperature via polymorphic phases in the lead-free (K,Na)NbO₃ ceramics

In this study, we report a lead-free piezoelectric ceramic system based on the composition (1-x)0.97(K_0.5Na_0.5)NbO₃-0.03LiTaO₃-xBiFeO₃, (KNNLT-xBF), which exhibits remarkable ferroelectric, piezoelectric, and dielectric properties. These enhancements in various properties are achieved by engineering a coexistence of orthorhombic and tetragonal phases close to room temperature. The incorporation of BF into the (KNNLT) composition shifts the orthorhombic-tetragonal mixed phase closer to room temperature. This effect enables the simultaneous realization of remarkable electrical performance and a moderate Curie temperature as well. Specifically, the sample with x = 0.012 exhibited a coexistence of orthorhombic and tetragonal phases near room temperature. This structural configuration resulted in excellent functional properties, including a piezoelectric coefficient (d₃₃) of 283 pC/N, remnant polarization (Pᵣ) of 22 µC/cm², and a relative dielectric constant (εᵣ) of 1464, and an electromechanical coupling factor (k_p) of 36 % respectively. The superior combination of these multifunctionalities is attributed to the proximity of the orthorhombic-tetragonal phase boundary close to room temperature, and the hybridization between Bi 6p orbitals and O 2p orbitals as well. The piezoelectric performance of this work provides a path for the development of new lead-free ceramics for real applications.