Quantum chemical calculations were performed for the F+ and F centers in the cubic and orthorhombic phases of this perovskite ferroelectric. It was shown that 1 or 2 electrons, respectively, of the F+ and F centers were considerably delocalized, even in the ground state of defects, over the two Nb atoms which were nearest to the O vacancy, and over other close atoms. They more closely resembled electron defects in partly covalent SiO2 crystals (so-called E'1 centers) than F-type centers in ionic MgO crystals. Two or three absorption bands were predicted to appear, depending upon the crystalline phase, for each of the defects. The calculated absorption energies for the F+ center were 0.3 to 0.4eV lower than those for the F center.

E.A.Kotomin, R.I.Eglitis, A.I.Popov: Journal of Physics - Condensed Matter, 1997, 9[22], L315-21