Record high-temperature piezoelectric performance in BNKT-based single crystals via local heterostructure design

As a promising lead-free alternative to Pb-based piezoelectric materials, (Bi,Na,K)TiO₃ (BNKT)-based single crystals have attracted increasing interest for high-performance electromechanical applications. In this work, we report a local heterostructure engineering strategy to achieves record high temperature piezoelectric performance in BNKT-based single crystals. MnO₂ was introduced into flux-grown (Bi_0.48Na_0.425K_0.055Ba_0.04)TiO₃ single crystals (BNKBT–Mn), resulting in pronounced nanoscale compositional/structural heterogeneity. This engineered local disorder dramatically modifies polarization dynamics and domain configurations: at room temperature, the modified crystals exhibit a large piezoelectric coefficient d₃₃ = 513 pC/N while retaining a relatively high depolarization temperature (T_d) of 148 °C. A maximum piezoelectric coefficient of 1257 pC/N is achieved at 147 °C, together with a large unipolar electrostrain of 1.24 % and an exceptionally high converse piezoelectric coefficient (d₃₃^*) of 1771 pm/V. Further analyses indicate that the giant high-temperature piezoelectric response is closely associated with a temperature-driven tetragonal (T)–pseudocubic (PC) phase transition, together with the presence of local heterostructures and the evolution of domain states. This study demonstrates that the local heterostructure design provides an effective route to simultaneously enhance room-temperature performance and unlock superior high-temperature piezoelectric functionality in lead-free BNKT-based single crystals.