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, 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 O atoms are displaced by 1.53% a₀ 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.