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

Transmission electron microscopy is used to acquire electron energy-loss spectra from phase-specific regions of Pu and U metal, PuO2 and UO2, and aged, self-irradiated Pu metal. The N-4,N-5 (4d -> 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., alpha-Pu, delta-Pu, and aged delta-Pu. Moreover, our results suggest that the metal-oxide bonds in UO2 and PuO2 are strongly covalent in nature and do not exhibit an integer valence change as would be expected from purely ionic bonding.