First-principles calculation of influences of La-doping on electronic structures of KNN lead-free ceramics

The electronic structures of lead-free piezoceramic (K_0.5Na_0.5)NbO₃ (KNN) and La-doped KNN ((K_0.5Na_0.5)_0.994La_0.006NbO₃) are studied by using first principles calculation on the basis of density functional theory (DFT). The results reveale that the piezoelectricity stems from strong hybridization between the Nb atom and the O atom. At the same time, the K or Na atoms are replaced by the La doping atoms, which brings about the anisotropic relaxation. The La doping reduces the forbidden band, at the same time it makes Fermi surfaces shift toward the energetic conduction band (CB) of KNN. With the increase of La-doping intent, the phase structure of KNN extends from O-phase to T-phase and improves the piezoelectric properties of KNN.