Evolution of phase structure and enhancement of piezoelectric properties in (K_0.5Na_0.5)_0.94Li_0.06NbO₃–SrZrO₃ lead-free ceramics

(Na_0.5K_0.5)_0.94Li_0.06NbO₃–xSrZrO₃ (NKLN–xSZ) lead-free piezoelectric ceramics with a composition near the rhombohedral–tetragonal (R–T) phase boundary and high Curie temperature T _c have been designed and prepared by conventional solid-state sintering. The phase transition behavior and composition-dependent piezoelectric properties of NKLN–xSZ lead-free system are investigated systematically. The R–T phase boundary is identified to lie in the composition range of 0.04 < x < 0.06 in NKLN–xSZ ceramics by XRD and temperature-dependent dielectric spectroscopy. The ceramics in the vicinity of the R–T phase boundary exhibit a strong composition dependent and significantly enhanced piezoelectric properties. A comprehensive study of d ₃₃ (197–220 pC/N) and T _c (310–349 °C) were conducted in the NKLN–xSZ ceramics with 0.04 < x < 0.06. The ceramic with x ~ 0.05 shows the optimum electrical properties: d ₃₃ ~ 220 pC/N, k _p  ~ 30%, ε _r  ~ 778, tan δ ~ 0.024, and T _c  ~ 331 °C. The good piezoelectric properties together with high T _c of the NKLN–xSZ ceramics make NKLN–xSZ a promising environmental friendly lead-free material system for a wide range of electromechanical transducer applications.