An intrinsic origin of enhanced piezoelectricity in KNN lead-free ceramics studied by first-principles calculations*

The intrinsic piezoelectricity in orthorhombic KN and KNN alloys has been investigated using first-principles methods. We found that the longitudinal piezoelectric coefficient  of orthorhombic KN and KNN piezoceramics was mainly contributed by the d₃₃ and d₁₅ of their single crystal. The calculated longitudinal piezoelectric coefficients are 52.7 pC/N and 89.3 pC/N in KN and KNN ceramics, respectively, which confirms the enhanced piezoelectricity in KNN piezoceramics. The enhanced piezoelectricity mainly comes from the increased response of internal coordinates of Na and O atoms to a macroscopic strain. The underlying mechanism is that the smaller atomic size of Na gives a larger free space for itself and adjacent O atoms in the perovskite structure, increasing the local structural instability. Our result provides a microscopic insight to understand the enhanced piezoelectricity in KNN piezoceramics.