Electronic structure of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">SrFe</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>and 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 ${\mathrm{\ensuremath{\Delta}}}_{\mathit{e}\mathit{f}\mathit{f}}$, 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 ${\mathrm{CaFeO}}_{3}$.