Quantum chemical modelling of point defects in KNbO3 perovskite crystals

We present results of semi-empirical quantum chemical calculations for several perovskite KNb_xTa_1?xO₃ (KTN) solid solutions, as well as point intrinsic defects – F centers and hole polarons bound to K vacancy – in KNbO₃. Method of the intermediate neglect of the differential overlap (INDO) was combined with typically 320-atom supercells and atomic geometry optimization. Analysis of the optimized atomic and electronic structure has clearly demonstrated that several nearest Nb atoms substituting for Ta in KTaO₃ – unlike Ta impurities in KNbO₃ – reveal the self-ordering effect, which probably triggers the ferroelectricity observed in KTN. We predict co-existence of one-site (atomic) and two-site (molecular) polarons with close absorption energies (≈1 eV). When available, the INDO results are compared with ab initio calculations. The relevant experimental data are discussed.