Structure, dielectric, and piezoelectric properties of K_0.5Na_0.5NbO₃-based lead-free ceramics

Lead-free ceramics based on the (1 - x)K_0.5Na_0.5NbO₃-xBi(Zn_0.5Ti_0.5)O₃ (KNN-BZT) system obtained via the conventional solid-state processing technique were characterized for their crystal structure, microstructure, and electrical properties. Rietveld analysis of X-ray diffraction data confirmed the formation of a stable perovskite phase for Bi(Zn_0.5Ti_0.5)O₃ substitutions up to 30 mol%. The crystal structure was found to transform from orthorhombic Amm2 to cubic Pm3̄m through mixed rhombohedral and tetragonal phases with the increase in Bi(Zn_0.5Ti_0.5)O₃ content. Temperature-dependent dielectric behavior indicated an increase in diffuseness of both orthorhombic to tetragonal and tetragonal to cubic phase transitions as well as a gradual shift towards room temperature. The sample with x ≈ 0.02 exhibited a mixed rhombohedral and orthorhombic phase at room temperature. A high-temperature X-ray diffraction study confirmed the strong temperature dependence of the phase coexistence. The sample with the composition 0.98(K_0.5Na_0.5NbO₃)-0.02(BiZn_0.5Ti_0.5O₃) showed an improved room temperature piezoelectric coefficient d ₃₃ = 109 pC/N and a high Curie temperature T _C = 383 °C.