Tuning the Phase Structure and Electrical Properties of BNKT‐Based Ceramics via Li ⁺ Doping

ABSTRACT Bi _0.5 [Na _{0.84‐ x} K _0.16 Li _x ] _0.5 TiO ₃ (BNKT‐ x Li) lead‐free piezoelectric ceramics were successfully prepared. The phase structure and domain engineering synergistic method were used to improve the piezoelectric properties, and the influence of Li ⁺ doping on its phase structure and electrical properties was studied. The ceramics are located at the MPB, in the process of introducing Li ⁺ , altering both the phase ratio and tetragonality (c _T /a _T ), as well as influencing grain size and leakage current. These factors together make ferroelectric and piezoelectric properties stronger. However, with the increase of Li ⁺ content, the long‐range ordered structure of ferroelectrics is disrupted, and the material gradually becomes a relaxation type, resulting in stronger electrostrain. Specifically, BNKT‐0Li exhibited a high depolarization temperature ( T _d = 143.5°C), while BNKT‐0.045Li demonstrated optimal piezoelectric properties ( d ₃₃ = 173 pC/N, T _d = 111.8°C), the Curie temperature T _m and dielectric constant ε _max are 332.9°C and 4675°C, respectively. The variable temperature ferroelectric test found that the material gradually changed from a ferroelectric state to a relaxation state during temperature increase. The maximum electrostrain of BNKT‐0.045Li near T _d can reach more than 0.3%, which maintains the high piezoelectric coefficient d ₃₃ , and makes T _d high enough, so that the operating temperature range of BNT ceramics is widened.