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

The crystal structures and associated energy band structures of classic lead-based PZT (PbZr_0.5Ti_0.5O₃) and lead-free KNN (K_0.5Na_0.5NbO₃) 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 PbZr_0.5Ti_0.5O₃ 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 PbZr_0.5Ti_0.5O₃ 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 PbZr_0.5Ti_0.5O₃ 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.