The electronic structure of SrFeO3 has been investigated by x-ray photoemission and ultraviolet photoemission spectroscopy. We find that the ground state consists of heavily mixed d4 and d5L states, reflecting the large covalency. The Fe 3s core-level splitting, together with a subsequent cluster-model configuration-interaction calculation, shows that a high-spin t2g3eg ground state is stabilized. The Fe 2p core levels have been interpreted using a p-d charge-transfer cluster-model calculation. The charge-transfer energy eff, defined with respect to the lowest multiplet levels of the d4 and d5L configurations, is negative, which means that a large amount of charge is transferred via Fe-O bonds from the O 2p bands to the metal d orbitals and that the ground state is dominated by the d5L configuration. This reduces the charge on the ionic sites, leading to only a small chemical shift between the Fe3+ and Fe4+ compounds. The band-gap energy Egap, calculated using the cluster model for the high-spin d4 configuration, is small due to the small charge-transfer energy and the large exchange stabilization of the adjacent d5 configuration. This small value for Egap leads to the presence of itinerant d electrons in the periodic lattice, causing metallic conductivity in SrFeO3 and charge disproportionation in CaFeO3.
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