Improvement in temperature stability and modified polymorphic phase transition of La-doped (Na _0.52K _0.44Li _0.04) Nb _0.8Ta _0.2O ₃ lead-free piezoelectric ceramics

Lead-free ceramics (Na _0.52K _0.44Li _0.04) _1-3xLa _xNb _0.8Ta _0.2O ₃ (KNLNT-Lax, x = 0.00-1.25 mol%) were prepared by traditional method. The piezoelectric proprieties and the polymorphic phase transition behaviors around the room temperature were investigated. The results show that high density (ρ ~ 95%) and outstanding piezoelectric performance (d ₃₃ ≥ 200 pC/N, k _p ≥ 40%) were obtained with 0.5-0.75 mol% La doping. By the research of temperature dependent ε _r, Δf _r/f _{r20 °C}, and k _p, we acquired the extended and mild polymorphic phase transition region, and obvious improvement of temperature stability in - 50-80 °C with the maximum derivation d(Δf _r/f _{r20 °C}) _max less than 2%, as the La doping content exceeds 0.5 mol%. The possible mechanism of enhanced properties and temperature stability was analyzed. Both excellent piezoelectric and temperature stability characteristics make the La-doped KNLNT ceramics feasible for practical applications.