Structure and electronic properties of lead-free KTa _{1- x}Nb _xO ₃: Accurate Wu-Cohen and screened-exchange study

We present first-principles calculations of structure and electronic properties of lead-free potassium tantalate niobate KTa _{1- x}Nb _xO ₃ (KTN) using both Wu-Cohen functional (WC) and screened-exchange local density approximation (sX-LDA). WC predicts more accurate the lattice constants and atomic structure of KTN over standard LDA and generalized gradient approximation (GGA-PBE). sX-LDA successfully corrects the band-gap problem of LDA and PBE. The correctness of sX-LDA is further supported by excellent accordance between calculated absorption spectra and experiment. We show that in paraelectric state Ta and Nb atoms have obvious off-center displacements, which agree with experimental results and suggest that KTN are not displacive but order-disorder materials. We found that Ta and Nb have little contribution to valance band-top states, and therefore optical properties of KTN are not sensitive to Ta/Nb disorder.