High piezoelectric performance and ergodic-nonergodic relaxor transition induced by single crystallization in Bi_0.5Na_0.5TiO₃-based materials

While the polar states of single crystals and ceramics are typically comparable, we demonstrate that single-crystallization markedly alters the polar behavior of Bi_0.5Na_0.5TiO₃-based (BNT) materials, resulting in significantly enhanced piezoelectric performance. The (Bi_0.465Na_0.44K_0.055Ba_0.04)(Ti_0.97Nb_0.03)O₃ (BNKBTN) single crystals exhibit a high piezoelectric coefficient of 674 pC/N and a unipolar strain of ∼0.29 % under 50 kV/cm, representing increases of ∼87 % and ∼97 % over BNKBT single crystals. These enhancements are attributed to a field-induced pseudocubic-to-tetragonal phase transition and an oxygen octahedral tilt evolution from a ⁰ a ⁰ a ⁰ to a ⁰ a ⁰ c ⁺. The BNKBTN single crystals exhibit a nonergodic relaxor state, whereas ceramics display weak piezoelectricity (∼45 pC/N) and an ergodic relaxor state at room temperature, likely due to grain size effects. Additionally, a strong frequency dependence in strain is observed near the depolarization temperature. At 100 ℃, the strain reaches ∼0.97 % with a converse piezoelectric coefficient of ∼1937 pm/V. A local strain increase near 50 ℃ correlates with the temperature-dependent in situ piezoelectric coefficient ( d ₃₃) response. These findings offer new insights into structure–property tuning in BNT-based lead-free piezoelectrics.