Abstract
We present results of semi-empirical quantum chemical calculations for several perovskite KNbxTa1−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.
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