Oxidation and aging in U and Pu probed by spin-orbit sum rule analysis: Indications for covalent metal-oxide bonds

Abstract

Transmission electron microscopy is used to acquire electron energy-loss spectra from phase-specific regions of Pu and U metal, $\mathrm{Pu}{\mathrm{O}}_{2}$ and $\mathrm{U}{\mathrm{O}}_{2}$, and aged, self-irradiated Pu metal. The ${N}_{4,5}$ $(4d\ensuremath{\rightarrow}5f)$ spectra are analyzed using the spin-orbit sum rule. Our results show that the technique is sensitive enough to detect changes in the branching ratio of the white-line peaks between the metal and dioxide of both U and Pu. There is a small change in the branching ratio between different Pu metals, and the data trends as would be expected for varying $f$ electron localization, i.e., $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$, $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}$, and aged $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}$. Moreover, our results suggest that the metal-oxide bonds in $\mathrm{U}{\mathrm{O}}_{2}$ and $\mathrm{Pu}{\mathrm{O}}_{2}$ are strongly covalent in nature and do not exhibit an integer valence change as would be expected from purely ionic bonding.