Phase structure and electrical properties of (Li,Ta)-doped (K,Na)NbO₃ lead-free piezoceramics in the vicinity of Na/K = 50/50

The phase structure and electrical properties of (K,Na)NbO₃ (KNN)-based lead-free piezoelectric ceramics with nominal composition Li_0.03 (Na_xK_1−x)_0.97Nb_0.8Ta_0.2O₃ (LKNNT, x = 0.50−0.55) were investigated with an emphasis on the influence of Na/K ratio. By XRD and Raman measurements, it was revealed that the phase transition from the co-existence of O₁ and T₁ to the co-existence of O₂ and T₂ occurs when x = 0.52. At this Na/K ratio, a peak of dielectric constants was obtained, which also corresponds to a morphotropic phase boundary between T₁ and T₂ besides that between O₁ and O₂ in LKNNT ceramics. All the Li_0.03 (Na_xK_1−x)_0.97Nb_0.8Ta_0.2O₃ ceramics with x = 0.50 − 0.55 show excellent piezoelectric performance, and the piezoelectric coefficient d*₃₃ can reach as high as 335 pm/V. The room-temperature piezoelectric properties in the present LKNNT ceramics are not sensitive to the change of Na/K ratio, indicating that the orthorhombic to tetragonal phase co-existence dominates the enhancement of piezoelectric properties. However, the results on the piezoelectricity measurement at elevated temperature showed possible to further enhance piezoelectric properties by adjusting Na/K ratio in the tetragonal LKNNT ceramics.