High Piezoelectric Performance and Phase Transition in Stressed Lead-Free (1 – x)(K, Na)(Sb, Nb)O₃-x(Bi, Na, K)ZrO₃ Thin Films

Although high performance piezoelectric properties have been reported in (K, Na)NbO₃-based bulk ceramics by constructing morphotropic phase boundary (MPB) with complex compositions, it is still challenging to achieve excellent piezoelectric properties in thin films with the same MPB compositions due to the serious volatile loss of the alkali constituents. Moreover, the stress due to substrate constraint also changes the film's crystal structure and shifts the film's MPB. Here this study demonstrates the highest ever reported effective piezoelectric strain coefficient d₃₃ of 184.0 pm V⁻¹ and voltage coefficient g₃₃ of 39.4 mm V N⁻¹ from macroscale characterization in a solution-derived lead-free piezoelectric thin film with a composition of (1 – x)(K, Na)(Sb, Nb)O₃-x(Bi, Na, K)ZrO₃ (KNSN-BNKZx, 0.01 ≤ x ≤ 0.07). With the effective suppression of volatile compositional loss by selecting appropriate combinational chemical agents in the precursor solution, phase transitions from orthorhombic, rhombohedral to tetragonal are observed experimentally and further analyzed theoretically with first principle simulation of the KNSN-BNKZx films, and the obtained coexistence of rhombohedral–tetragonal phase at x = 0.05 contributes to the outstanding piezoelectric performance in the tensile stressed films. The results demonstrate a valuable strategy for realizing high-performance piezoelectric properties in thin films with volatile and complex MPB compositions under stress condition.