Li/Mn co-doped K_0.5Bi_4.5Ti₄O₁₅ high-temperature piezoelectric ceramics: Structures, properties, and ultrasonic transducer applications

K_0.5Bi_4.5Ti₄O₁₅ (KBT, Curie temperature T_C = 558 °C), a representative Aurivillius-type ferroelectric material, is considered a promising candidate for high-temperature piezoelectric devices. In this study, the co-doping strategy was adopted by introducing Li/Mn ions into the A-site of KBT, and a series of polycrystalline ceramics with the formula (KBi)_0.5−x(LiMn)_xBi₄Ti₄O₁₅ (x = 0–0.065, abbreviated as KBT–LM1000x) were synthesized via the conventional solid-state reaction method. The substitution mechanisms of Li/Mn ions and their doping concentration effects on the crystalline structure, defect chemistry, and dielectric/ferroelectric properties were investigated. The synergistic substitution of Li/Mn for K/Bi induced a transition of KBT in its dielectric behavior from a sharp λ-type anomaly to a diffuse or relaxor-like response, with both diffuseness parameter γ and T_C values gradually increasing with increasing x. The Li/Mn co-doping strategy effectively suppressed the leakage conduction of KBT, achieving balanced control of conduction suppression and domain-wall mobility. Among the systems, KBM55 achieved the highest piezoelectric coefficient (d₃₃ = 30 pC/N) and planar electromechanical coupling factor (k_p = 6.4%), as well as a higher Curie temperature (T_C = 590 °C). Furthermore, an ultrasonic transducer was fabricated by using this sample as conversion elements, which demonstrated a center frequency (f_c) of 2.24 MHz, a −6 dB bandwidth (BW_{–6 dB}) of 33.95%, and stable pulse–echo signals up to 250 °C.