ELECTRONIC-STRUCTURES OF NIO, COO, AND FEO STUDIED BY 2P CORE-LEVEL X-RAY PHOTOELECTRON-SPECTROSCOPY

We have analyzed the satellite structures of the transition-metal 2p3/2 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-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 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.