Li-substituted K_0.5Na_0.5NbO₃-based piezoelectric ceramics: Crystal structures and the effect of atmosphere on electrical properties

Crystal structures and electrical properties of (Na_0.5K_0.5)_1-xLi_xNbO₃ (x = 0, 0.055, 0.06, 0.065, 0.07; KNN-xLi) ceramics were investigated systematically. Rietveld refinements confirmed that the ceramic with x = 0.06 modification had a typical morphtropic phase boundary (MPB) structure, where the proportions of orthorhombic (Amm2) and tetragonal (P4mm) phases were 58.37% and 41.63%, respectively. However, tetragonal phase structure was predominant in the x = 0.07 ceramic with 75.57% P4mm phase. Piezoelectric ceramics KNN-xLi were post-annealed in nitrogen (N₂) and oxygen (O₂), and atmosphere had a strong impact on the electrical properties of them. O₂ treatment resulted in improvements in ferroelectric polarization, electrostriction characteristic and piezoelectric property, due to the decrease in oxygen vacancy concentration. On the contrary, a deteriorated performance was found in the N₂-treated samples. Impedance data of KNN-xLi ceramics were largely dominated by the sample bulk, and variable-atmosphere experiments suggested that the KNN-0.06Li ceramic was mainly n-type material. The O₂-treated KNN-0.06Li ceramic with a T_c of 473 °C had a remnant polarization (P_r) of 29.2 μC/cm², a field-induced strain (S₃₃) of 0.152%, a piezoelectric coefficient (d₃₃) of 218 pC/N and an electromechanical coupling factor (k_p) of 43%.