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Zusammenfassung

We have investigated the electronic structure of the high oxidation state material YCrO 4 within the framework of the Zaanen-Sawatzky-Allen phase diagram. While Cr 4+-based compounds such as SrCrO 3/CaCrO 3 and CrO 2 can be classified as small-gap or metallic negative-charge-transfer systems, we find using photoelectron spectroscopy that YCrO 4 is a robust insulator despite the fact that its Cr ions have an even higher formal valence state of 5+. We reveal using band-structure calculations that the tetrahedral coordination of the Cr 5+ ions in YCrO 4 plays a decisive role, namely to diminish the bonding of the Cr 3d states with the top of the O 2p valence band. This finding not only explains why the charge-transfer energy remains effectively positive and the material stable, but also opens up a new route to create doped carriers with symmetries different from those of other transition-metal ions.