Owing to the complex composition architecture of these solid solutions, some fundamental issues of the classical (1−x)Bi_1/2Na_1/2TiO-xBi_1/2K_1/2TiO₃ (BNT-xBKT) binary system, such as details of phase evolution and optimal Na/K ratio associated with the highest strain responses, remain unresolved. In this work, we systematically investigated the phase evolution of the BNT-xBKT binary solid solution with x ranging from 0.12 to 0.24 using not only routine X-ray diffraction and weak-signal dielectric characterization, but also temperature-dependent polarization versus electric field (P-E) and current versus electric field (I-E) curves. Our results indicate an optimal Na/K ratio of 81/19 based on high-field polarization and electrostrain characterizations. As the temperature increased above 100 °C, the x = 0.19 composition produces ultrahigh electrostrains (> 0.5%) with high thermal stability. The ultrahigh and stable electrostrains were primarily due to the combined effect of electric-field-induced relaxor-to-ferroelectric phase transition and ferroelectric-to-relaxor diffuse phase transition during heating. More specifically, we revealed the relationship between phase evolution and electrostrain responses based on the characteristic temperatures determined by both weak-field dielectric and high-field ferroelectric/electromechanical property characterizations. This work not only clarifies the phase evolution in BNT-xBKT binary solid solution, but also paves the way for future strain enhancement through doping strategies.

Low-temperature sintered (Na_1/2Bi_1/2)_0.935Ba_0.065Ti_0.975(Fe_1/2Nb_1/2)_0.025O₃ (NBT-BT-0.025FN) lead-free incipient piezoceramics were investigated using high-purity Li₂CO₃ as sintering aids. With the ≤0.5 wt% Li₂CO₃ addition, the introduced Li⁺ cations precede to enter the A-sites of the perovskite lattice to compensate for the A-site deficiencies. Once the addition exceeds 0.5 wt%, the excess Li⁺ cations will occupy B-sites and give rise to the generation of oxygen vacancies, which accelerate the mass transport and thus lower the sintering temperature effectively from 1100 ℃ down to 925 ℃. It was also found that a small amount of Li⁺ addition has little effect on the phase structure and electromechanical properties of the system, but overweight seriously disturbs these characteristics because of the large lattice distortion. The sintered NBT-BT-0.025FN incipient piezoceramics with 1.25 wt% Li₂CO₃ addition at 925 ℃ provides a large strain of 0.33% and a corresponding large signal piezoelectric coefficient d₃₃ of 550 pm/V at 60 kV/cm, indicating this system is a very promising candidate for lead-free co-fired multilayer actuator application.