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 ca lculations for a series of perovskite KNb3Ta1-xO3 (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 ce ll (LUC) periodic model is used. According to the INDO calculations, Nb imp urity becomes off-center in KTaO3 already at the lowest studied concentrati ons (x = 0.125), in a good agreement with XAFS measurements. We compare our results with previous ab initio FP-LMTO calculations. Quantum chemical cal culations confirm the existence of self-trapped electrons in KNbO3: the cor responding lattice relaxation energy is 0.21 eV. We estimate the optical ab sorption energy to be 0.78 eV. An electron in the ground state occupies a t (2g) orbital of the Nb4- ion. Its orbital degeneracy is lifted by a combina tion of the breathing and Jahn-Teller (JT) modes when four nearest equatori al 0 atoms are displaced by 1.4% a(o) outwards and two oxygens shift 1% inw ards along the z axis. (C) 2001 Elsevier Science B.V. All rights reserved.