Insight for the construction of R-T phase boundary in KNN piezoceramics from the view of energy band structure and electron density

Abstract The crystal structures and associated energy band structures of classic lead-based PZT (PbZr0.5Ti0.5O3) and lead-free KNN (K0.5Na0.5NbO3) piezoelectric ceramics prepared by a solid-state method are systematically studied based on the First-principles calculations. For better understanding relations between energy band structures and the phase structures in the PbZr0.5Ti0.5O3 and KNN, the components PT (PbTiO3)/PZ (PbZrO3) and KN (KNbO3)/NN (NaNbO3) are also calculated. The results suggest that the vital contribution to R-T morphotropic phase boundary (MPB) in PbZr0.5Ti0.5O3 comes from the hybridization of A-site and B-site ions, which causes stretching and tilting of oxygen octahedron and therefore leads to the formation of R and T phases. The KNN behaves differently. Moreover, it's concluded that the band structure highly inherited from tetragonal PbTiO3 plays an important role in the construction of the R-T phase boundary according to the comparison of PbZr0.5Ti0.5O3 and KNN. In general, the work inspires an idea from the view of energy band structure to realize R-T phase coexistence as well as improved piezoelectric properties in KNN-based lead-free piezoceramics.