Ab Initio Hybrid Dft Calculations Of Batio3 Bulk And Bao-Terminated (001) Surface F-Centers

Using a supercell model and a hybrid B3PW exchange-correlation functional, we have performed first principles calculations for the F-center in the BaTiO3 bulk and on the BaO-terminated (001) surface. We find that two Ti atoms nearest to the bulk F-center are repulsed, while nearest eight oxygen and four barium atoms relax toward the oxygen vacancy (by 1.06, 0.71 and 0.08% of the lattice constant ao, respectively). The magnitudes of atomic displacements around the F-center located on the BaO-terminated (001) surface in most cases (except for Ti) are larger than those around the bulk F-center (0.1, 1.4 and 1.0% of ao, respectively). Our calculated BaTiO3 bulk Gamma-Gamma bandgap of 3.55 eV is in an acceptable agreement with the respective experimental bandgap value of 3.2 eV. The pristine BaO-terminated (001) surface F F bandgap (3.49 eV) is reduced with respect to the bulk bandgap value. The bulk and BaO-terminated (001) surface F-center bands in BaTiO3 matrix are located only at 0.23 eV and 0.07 eV under the conduction band (CB) bottom, indicating that the F-center is a shallow donor. The F-center in the BaTiO3 bulk contains charge of 1.103e, whereas slightly less charge, only 1.052e, are localized inside the F-center on the BaO-terminated (001) surface. Our calculations demonstrate considerable increase of the chemical bond covalency between the BaTiO3 bulk F-center and its two nearest Ti atoms equal to 0.320e, and even larger increase for BaO-terminated (001) surface F-center and its nearest Ti atom 0.480e, in comparison to the relevant Ti-O chemical bond covalency in the perfect BaTiO3 bulk 0.100e. The difference between F-center formation energy in BaTiO3 bulk (10.3 eV) and on the BaO-terminated (001) surface (10.2 eV) trigger the segregation of the F-center from the bulk toward the BaO-terminated (001) surface.