Quantum chemical modelling of electron polarons and charge-transfer vibronic excitons in BaTiO3 perovskite crystals

As an extension of our previous study on the electron polarons and excitons in KNbO3 and KTaO3 [1, 2], we present here results of semi-empirical intermediate-neglect-of-differential-overlap (INDO) calculations for free electron polarons, single-triplet excitons and the excitonic phase in BaTiO3 perovskite crystal. Our INDO calculations confirm the existence of self-trapped electrons in BaTiO3. The corresponding lattice relaxation energy is 0.24 eV and the optical absorption energy 0.69 eV. An electron in the ground state occupies the t(2g) orbital of the Ti3+ ion. Its orbital degeneracy is lifted by a combination of the breathing and Jahn-Teller modes when four nearest equatorial 0 atoms are displaced by 1.53% a(0) outwards in the x-y plane and another two nearest oxygens shift 1.1% inwards, along the z-axis. Our INDO calculations show that creation of charge-transfer vibronic exciton (CTVE) in BaTiO3 Crystal is accompanied by a strong lattice distortion; the relevant energy gain due to CTVE formation is 2.2 eV. Moreover, our INDO calculations predict the existence of a new crystalline phase-that of CTVEs in BaTiO3 where strongly correlated CTVEs are located in each unit cell of a crystal.