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Abstract:
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.
