Phase structure and piezoelectric properties of Na_0.5K_0.5 NbO₃LiTaO₃ lead-free ceramics

(1-x) Na_0.5K_0.5NbO_3-xLiTaO₃ lead-free piezoelectric ceramics were prepared and the effects of LiTaO₃ on crystal structure and piezoelectric properties were investigated. The results reveal that the perovskite structures of material will transform from orthorhombic phase to tetragonal phase as LiTaO₃ content increases from 0.04 mol to 0.10 mol. The composition of 0.04 mol < x < 0.08 mol is correspond to morphotropic phase boundary (MPB) between orthorhombic and tetragonal phase. The ferroelectric phase transitional temperature (T_T-C) of tetragonal to orthorhombic will shift to lower temperature (< 150°C) while the curie temperature (T_C) will shift to higher temperature (> 450°C) because of increasing LiTaO₃ content. The samples with composition 0.94 Na_0.5K_0.5 NbO₃-0.06LiTaO₃ exhibit optimal piezoelectric properties: relative density is 97.8%, piezoelectric constant (d₃₃) is 134 pC/N, electromechanical coupling (k_p) is 0.33, dielectric constant (ε) is 897, mechanical quality factor (Q_m) is 100 and curie temperature (T_C) is 480°C.