Intrinsic origin of enhanced piezoelectricity in alkali niobate-based lead-free ceramics

An orientational average method is used to calculate the piezoelectric coefficients of KNbO3 and K0.5Na0.5NbO3 (KN and KNN) piezoceramics, which is the first time that an ab initio method has been used to directly compare the piezoelectric coefficients with the experimental results of the orthorhombic alkali niobate-based lead-free piezoceramics. It is found that the longitudinal piezoelectric coefficient d33 of orthorhombic KN and KNN piezoceramics is mainly contributed by d(33) and d(15) of their single crystal. The approximately 70% increase in the calculated d33 is confirmed in pure orthorhombic KNN over KN piezoceramics. The increased response of the internal coordinates of Na and O atoms to macroscopic strain is considered the main origin of the enhanced piezoelectricity. The underlying mechanism is that the smaller atomic size of Na provides more room for itself and the adjacent O atoms in the perovskite structure, which leads to the increased local structural instability. In this case, the Na and its nearby O atoms are easier to move when an external stress is applied on the KNN and then induce the substantial change in polarization. This work provides a microscopic insight to understand the enhanced piezoelectricity in KNN piezoceramics.