Electronic structures of NiO, CoO, and FeO studied by 2p core-level x-ray photoelectron spectroscopy.

Abstract

We have analyzed the satellite structures of the transition-metal 2${\mathit{p}}_{3/2}$ core-level photoemission spectra of NiO, CoO, and FeO within the charge-transfer cluster model. The spectra can be fitted with reasonable values of the parameters of the model, which show the expected trend along the transition-metal series as well as along the ligand series including halogen and oxygen. The values of the d-d electron correlation energy U and the charge-transfer energy \ensuremath{\Delta} from the ligand to the metal 3d level in the valence electronic structures are then estimated from these parameter values. We confirm that the large-U value is important for the insulating properties of these monoxides, but the band gap is not determined by U, but rather by \ensuremath{\Delta}. This supports the recent proposal that these oxides are not Mott-Hubbard insulators in the original sense, but belong to the charge-transfer-type insulators. We also calculate the magnitudes of the band gaps with our parameter values, and they are in fair agreement with experimental values and show the correct trend.