Influence of compositional ratio K/Na on physical properties in (KxNa1−x)NbO3 ceramics

Lead-free KxNa1−xNbO3 (KNN) ceramics with x=0.10–0.80 were prepared by a conventional solid-state reaction method to investigate the influence of the K/Na ratio on their physical properties. It has been found that the KNN ceramics with x=0.45–0.60 exhibit quite different microstructures from those with other alkali compositional ratios. The lattice parameter discontinuity previously reported at x=0.475 has not been confirmed. The existence of a change in the line slopes of lattice parameter variation with x is verified in the vicinity of x=0.35. The permittivity ε′, piezoelectric constant d33, and planar electromechanical coupling coefficient kp show broad peaks in the compositional range of x=0.40–0.60, where d33 and kp are nearly independent of the K/Na ratio and all the ceramics have the high piezoelectric values of d33∼125 pC/N and kp∼0.41, respectively. From the analyses, we suggest that the high piezoelectric properties for the KNN ceramics near x=0.50 are closely related to their microstructures of well-faceted and uniformly distributed large grains rather than the widely believed morphotropic phase boundary effect of lattice parameter discontinuity.