Charge distribution and optical properties of F⁺ and F centres in KNbO₃ crystals

Results of quantum chemical calculations for the F⁺ and F centres in cubic and orthorhombic phases of a KNbO₃ perovskite ferroelectric are presented and analysed in the light of existing experimental literature. It is shown that one (two) electrons of the F⁺ and F centres, respectively, are considerably delocalized, even in the ground state of defects, over the two Nb atoms nearest to the O vacancy, and other close atoms. They resemble more electron defects in partly covalent SiO₂ crystals (the so-called E'₁ centre) than F-type centres in ionic MgO crystals. We predict two or three absorption bands (depending on the crystalline phase) for each of the defects. The calculated absorption energies for the F⁺ centre are by 0.3–0.4 eV lower than those for the F centre.