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The electronic structure of SrTi03 has attracted much attention due to its perovskite structure and its surface chemical activity. Perovskite-type compounds are of special interest with regard to the structure-related high-T, superconductors. We have focused our attention on the effective charges of the cations and the anion, which are reduced relative to their formal ionic charges due to the covalent bonding between 0 2p and Ti 3d electrons. Using photoelectron (h v=100 eV) corelevel shifts observed on Ti02and SrO-plane-terminated (001) surfaces and shifts due to oxygen vacancies ( Vo), we were able to identify Ti and Sr surface cations and reduced Ti cations in Ti-Vo complexes. Within a simple "localized-hole point-ion" model, the comparison of the experimental binding energies with calculated ionization energies of free ions in different valence states modified by the corresponding Madelung potentials enables us to deduce the effective charges on the Ti ions to be about +2.5 (3d" orbital occupation) in the bulk and about +2.0 (3d') on the surface, respectively. This conclusion is drawn from the calculated variation of the point-ion energies with the valence-orbital occupation numbers (Ti 3d", Sr 5s, and 0 2p ). The free-ion ionization energies have been obtained from self-consistent-field atomic-structure calculations. Our results for the degree of covalency in the bulk and at the surface are in very good agreement with recent band calculations for the transition metal SrTiO, . This further shows that reliable information about the ground state of a solid can be drawn from core-level spectroscopy.

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