Quantum chemical modelling of polarons and perovskite solid solutions

Following our previous study [J. Phys.: Condens. Matter 10 (1998) 6271] of a single Nb impurity and Nb clusters in KTaO₃, we present results of the calculations for a series of perovskite KNb_xTa_{1 - x}O₃ (KTN) solid solutions (x = 0, 0.125, 0.25, 0.75, 1). The quantum chemical method of the intermediate neglect of the differential overlap (INDO) combined with the large unit cell (LUC) periodic model is used. According to the INDO calculations, Nb impurity becomes off-center in KTaO₃ already at the lowest studied concentrations (x = 0.125), in a good agreement with XAFS measurements. We compare our results with previous ab initio FP-LMTO calculations. Quantum chemical calculations confirm the existence of self-trapped electrons in KNbO₃; the corresponding lattice relaxation energy is 0.21 eV. We estimate the optical absorption energy to be 0.78 eV. An electron in the ground state occupies a t_2g orbital of the Nb⁴⁺ ion. I ts orbital degeneracy is lifted by a combination of the breathing and Jahn-Teller (JT) modes when four nearest equatorial O atoms are displaced by 1.4% a_o outwards and two oxygens shift 1% inwards along the z axis.