Mott-Wannier excitons in the tetragonal BaTiO₃ lattice

Structural and electronic properties of excitons in the tetragonal BaTiO₃ crystal is studied using a quantum chemical method developed for crystals. The obtained defect structure corresponds to the so-called Mott-Wannier-type exciton having a considerable distance between the hole and electronic parts of the defect. Performed crystalline lattice relaxation shows displacements of atoms in an extensive defective region of up to 12 atoms. However, the calculated magnitudes of atomic movements are not large, normally not exceeding 0.08 Å. It is also observed that the self-trapped exciton polarizes the lattice around it. Using the so-called △SCF method, the luminescence energy due to the exciton is found to be equal to 0.94 eV.