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.

• Received 5 July 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.2339