Theoretical study of structural and optical properties of F-centers in tetragonal BaTiO3

Using the density functional theory (DFT) within the local density approximation (LDA) and a method based on the Hartree-Fock (HF) approximation, we study the structural, electronic and optical properties of F-centers (two electrons in an oxygen-vacancy) in the tetragonal lattice of the perovskite-type BaTiO3 crystal. In this structure the F-center has two non-equivalent positions due to the two different O atoms; namely, the F-center situated in the Ti-O-Ti chains along the [0011 polarization axes (V-a(0)) and the F-center situated in the Ti-O-Ti chains within the x-y plane (V-b(0)). Both cases are considered in our study. The obtained results point out that the crystal structure containing V-a(0) centre is energetically more stable than that of the V-b(0) center. The two electrons of the F-center are found to be localized on the two adjacent Ti ions and their ground state is composed mainly of Ti 3d(z)(2) atomic orbital. Besides, we have computed the allowed optical transitions for the F-center using the so-called DeltaSCF method. As a result the obtained absorption energies are found to be equal to 1.47 eV for the V-a(0) center as well as 3.07 and 4.27 eV for the V-b(0) center.