Quantum chemical modelling of electron polarons and excitons in ABO₃ perovskites

Quantum chemical calculations using the intermediate neglect of the differential overlap (INDO) method, combined with the large unit cell periodic model argue for an existence of the self-trapped electrons in KNbO₃ and KTaO₃ perovskite crystals. An electron in the ground state occupies predominantly t_2(g) orbital of a Nb⁴⁺ ion. Its orbital degeneracy is lifted by a combination of the breathing and Jahn-Teller modes where four nearest equatorial O atoms are displaced outwards and two oxygens shift inwards along the z axis. Triplet exciton is shown to be in a good approximation of a pair of nearest Jahn-Teller electron and hole polarons (a bipolaron) which is very likely responsible for the 'green' luminescence observed in these crystals.