Effects of coupling ability of dopants on NbO6 vibration and piezoelectric properties of KNN lead-free ceramics

In the work, the lead-free (K0.5Na0.5)NbO3, (K0.5Na0.5)(0.95)Li/Ag0.05NbO3 and (K0.5Na0.5)Nb0.95Sb/Ta0.05O3 piezoelectric ceramics were prepared by a solid-state method. The relationships between the coupling ability of the dopants and NbO6 vibration of the ceramics were investigated. The intensity of NbO6 vibration is ascribed to the dopant's radius. However, the distortion of NbO6 is independent of the vibration. The dopants with high coupling ability (i.e., Ag and Ta) give rise to more severe distortion of NbO6, resulting in bigger piezoelectric enhancement for KNN-Ag and KNN-Ta. Furthermore, the NbO6 vibration plays an indispensable role in ferroelectric phase transition while the c/a accounts for the Curie temperature.