Electronic structure ofSrFe4+O3and related Fe perovskite oxides

The electronic structure of ${\mathrm{SrFeO}}_3$ has been investigated by x-ray photoemission and ultraviolet photoemission spectroscopy. We find that the ground state consists of heavily mixed ${\mathit{d}}^4$ and ${\mathit{d}}^5$L 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 ${\mathit{t}}_{2\mathit{g}}^3$ ${\mathit{e}}_{\mathit{g}}$ 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 ${\mathrm{\Delta }}_{\mathit{e}\mathit{f}\mathit{f}}$, defined with respect to the lowest multiplet levels of the ${\mathit{d}}^4$ and ${\mathit{d}}^5$L 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 ${\mathit{d}}^5$L configuration. This reduces the charge on the ionic sites, leading to only a small chemical shift between the ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{4+}$ compounds. The band-gap energy ${\mathit{E}}_{\mathrm{gap}}$, calculated using the cluster model for the high-spin ${\mathit{d}}^4$ configuration, is small due to the small charge-transfer energy and the large exchange stabilization of the adjacent ${\mathit{d}}^5$ configuration. This small value for ${\mathit{E}}_{\mathrm{gap}}$ leads to the presence of itinerant d electrons in the periodic lattice, causing metallic conductivity in ${\mathrm{SrFeO}}_3$ and charge disproportionation in ${\mathrm{CaFeO}}_3$.