Ferroelectric and piezoelectric properties of Bi_0.5(Na_{0.925−x−y}Li_0.075K_xAg_y)_0.5TiO₃ lead-free ceramics

Lead-free ceramics Bi_0.5(Na_{0.925−x−y}Li_0.075K_xAg_y)_0.5TiO₃ have been prepared by an ordinary sintering technique, and their ferroelectric and piezoelectric properties have been studied. From the results of x-ray diffraction, Li⁺, K⁺ and Ag⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) exists at 0.15 < x < 0.25. Compared with pure Bi_0.5Na_0.5TiO₃, the substitutions of K⁺ and Ag⁺ lower the coercive field E_c greatly and increase the remanent polarization P_r of the ceramics. Because of the MPB and low E_c, the piezoelectricity is significantly enhanced. For the ceramics with compositions near the MPB, the piezoelectric coefficient d₃₃ = 178–219 pC N⁻¹, the planar electromechanical coupling factors k_P = 35–39% and k_t = 44–51%. Also, the depolarization temperature T_d reaches a minimum value near the MPB. The results of the ferroelectric and dielectric properties at high temperatures suggest that the ceramics may contain both the polar and non-polar regions at temperatures above T_d.