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 3 d(0) (TiO2, V2O5) to 3d(10) (Cu2O). The photoemission core-level spectra ar e 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 ene rgy to the hybridization energy significantly decreases from the late to th e early transition metal oxides. This trend is attributed mostly to the inc reasing number of empty d states in the early transition metals which enhan ces the effective metal-ligand hybridization. We also compare the experimen tal valence band spectra with densities of states (DOS) from band-structure calculations. The rather good agreement between the theoretical DOS and th e measured single-particle excitation spectra of the early 3d-transition-me tal oxides as opposed to the failure of the one-electron description for mo st of the late transition metal oxides supports the results of the cluster model analysis.