Magnitude and Origin of the Band Gap in NiO

Photoemission and bremsstrahlung-isochromat-spectroscopy data on a cleaved NiO single crystal are presented and compared to band- and cluster-theory predictions. In contrast to band-theory predictions the band gap is found to be large but not determined solely by the even larger d-d Coulomb interactions so that NiO is not a Mott-Hubbard insulator in the simplest sense. A large d-d interaction need not prevent NiS from being a metal.