Li-substituted K0.5Na0.5NbO3-based piezoelectric ceramics: Crystal structures and the effect of atmosphere on electrical properties

Abstract Crystal structures and electrical properties of (Na 0.5 K 0.5 ) 1− x Li x NbO 3 ( x  = 0, 0.055, 0.06, 0.065, 0.07; KNN- x Li) 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 ( Amm 2) and tetragonal ( P 4 mm ) phases were 58.37% and 41.63%, respectively. However, tetragonal phase structure was predominant in the x  = 0.07 ceramic with 75.57% P 4 mm phase. Piezoelectric ceramics KNN- x Li were post-annealed in nitrogen (N 2 ) and oxygen (O 2 ), and atmosphere had a strong impact on the electrical properties of them. O 2 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 N 2 -treated samples. Impedance data of KNN- x Li 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 O 2 -treated KNN-0.06Li ceramic with a T c of 473 °C had a remnant polarization ( P r ) of 29.2 μC/cm 2 , a field-induced strain ( S 33 ) of 0.152%, a piezoelectric coefficient ( d 33 ) of 218 pC/N and an electromechanical coupling factor ( k p ) of 43%.