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.