Microstructure and electrical properties of La-modified K0.5Na0.5NbO3 lead-free piezoelectric ceramics

Lead-free ceramics (K0.5Na0.5)(1-3x)LaxNbO3 (0 <= x <= 0.0175) have been fabricated by a conventional sintering technique. The results of XRD show that the ceramics possess a perovskite structure with orthorhombic symmetry. Moreover, doping inhibits the grain growth, decreases the ferroelectric-paraelectric phase transition temperature and induces a diffuse phase transition. At low doping levels (x <= 0.0075), the observed remanent polarization (P-r) and coercive field (E-c) remain almost unchanged. As x increases from 0.0075 to 0.0175, P-r starts to decrease while E-c increases. Nevertheless, due to the increase in relative permittivity, the ceramic with x = 0.0125 exhibits the optimum piezoelectric properties, giving a large piezoelectric coefficient (d(33) = 135 pC N-1) and a high planar electromechanical coupling coefficient (k(P) = 0.40).