Quantum-chemical study of excitons in tetragonal BaTiO3 and SrTiO3 crystals

Using a quantum-chemical INDO method based on the Hartree-Fock formalism and the periodic large unit cell (LUC) model we present a theoretical interpretation of the structural and electronic properties of triplet excitons in the tetragonal BaTiO3 and SrTiO3 crystals. Our study demonstrates that the exciton structure has particularities in each material. In the BaTiO3 the defect structure corresponds to the so-called Mott-Wannier-type exciton having a considerable separation, 7.0 Angstrom, between the hole and the electron. Meanwhile, in SrTiO3 the structural and electronic features of the triplet exciton are quite different. The hole-electron distance is about 2.14 Angstrom and the defect is well localized in two contiguous atoms: the hole on one of the O atoms and the electron on the neighbor Ti atom. The calculated luminescence energy using the so-called DeltaSCF method is found to be equal to 0.94 eV and 1.13 eV for BaTiO3 and SrTiO3, respectively. Since it falls within the infrared part of the spectrum, the experimentally detected green luminescence due to photo-excited states should be attributed to the singlet excitons.