Electronic structure of 3d-transition-metal oxides:: on-site Coulomb repulsion versus covalency

We have performed photoemission and inverse photoemission experiments on a series of 3d-transition-metal oxides with formal ionic configuration from 3d(0) (TiO2, V2O5) to 3d(10) (Cu2O). The photoemission core-level spectra are analysed in terms of a simple cluster model leading to estimates for the charge-transfer energy Delta, the Coulomb correlation energy U-dd, and the hybridization strength V. It is found that the ratio of the correlation energy to the hybridization energy significantly decreases from the late to the early transition metal oxides. This trend is attributed mostly to the increasing number of empty d states in the early transition metals which enhances the effective metal-ligand hybridization. We also compare the experimental valence band spectra with densities of states (DOS) from band-structure calculations. The rather good agreement between the theoretical DOS and the measured single-particle excitation spectra of the early 3d-transition-metal oxides as opposed to the failure of the one-electron description for most of the late transition metal oxides supports the results of the cluster model analysis.