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.