Calculations of F centers in KNbO₃ ferroelectric crystals

Semi-empirical method of the intermediate neglect of the differential overlap (INDO) combined with the supercell model is applied to the calculations of the F center optical properties in ferroelectric KNbO₃ perovskite crystals. It is shown that two electrons of the defect are weakly localized inside the O vacancy, unlike similar defects in ionic alkali halides, but are considerably spread over two nearest Nb atoms. For the orthorhombic phase stable at room temperatures three absorption bands are predicted to be at 2.72 eV, 3.04 eV and 3.11 eV, respectively. The first energy is close to the band at 2.7 eV observed in electron- irradiated crystals. In the high temperature, cubic phase only two bands, 2.67 eV and 3.04 eV, are expected to arise due to higher local symmetry of the defect.