  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 like 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.
