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 Å, 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 Å 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 ΔSCF 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.