We have performed x-ray absorption experiments on uranium dioxide (UO${}_2$) at the O $1s$, U $4d$, U $4f$, and U $5d$ edges. After comprehensive energy calibrations for O $1s$, U $4d$, and U $4f$ spectra, we have used the U $4d$ and $4f$ spectra to sort the energetic positions of the $5f$ and the $6d$ states in the unoccupied band unambiguously. This demonstrates conclusively that UO${}_2$ is an $f$-$f$ Mott-Hubbard insulator, where the electronic repulsion between $f$ electrons is responsible for the insulating state. Calculations performed within the U-corrected generalized gradient approximation of the optical response of UO${}_2$ permit direct comparison with the absorption spectra and confirm the experimental results.

• Received 25 January 2011

DOI:https://doi.org/10.1103/PhysRevB.83.165102