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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 N4,5 (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., α-Pu, δ-Pu, and aged δ-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.
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Moore et al. (2006) studied this question.
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