Quantum chemical modelling of electron polarons and excitons in ABO(3) perovskites

Quantum chemical calculations using the intermediate neglect of the differe ntial overlap (INDO) method, combined with the large unit cell periodic mod el argue for an existence of the self-trapped electrons in KNbO3 and KTaO3 perovskite crystals. An electron in the ground state occupies predominantly t(2g) orbital of a Nb4+ ion. Its orbital degeneracy is lifted by a combina tion of the breathing and Jahn-Teller modes where four nearest equatorial O atoms are displaced outwards and two oxygens shift inwards along the z axi s. Triplet exciton is shown to be in a good approximation of a pair of near est Jahn-Teller electron and hole polarons (a bipolaron) which is very like ly responsible for the 'green' luminescence observed in these crystals.