Theoretical study of structural and optical properties of F-centers in tetragonal BaTiO_3

摘要

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 BaTiO_3 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 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~0center. 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 ASCF 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.