The fluorite structured binary oxides of the light actinides are central to all aspects of their nuclear fuel application. Twenty years ago their chemistry was “known.” The binary U-O phase diagram was perhaps the most complicated of any element, with &gt; 20 phases across UOx with x continuous between 3/2 and 3. In contrast, NpO2+x did not exist, resulting in a gap between NpO2 and the highest valence Np2O5, and Pu terminated at the (IV) state of PuO2. Then, in 2000, PuO2+x with x ≤ 0.26 was confirmed, with a similar result just this year for NpO2+x, demonstrating that mixed valence and disorder are displayed for all three actinides and not just U. The question, then, is whether the An(V) within the oxide behaves analogously to its corresponding species in molecular complexes or its presence in the lattice modifies its chemical properties. The corollary is to what extent UO2+x, NpO2+x, and PuO2+x are similar differ from each other? The proposed cuboctahedral or quad- and di-interstitial structures for U4O9 derived, respectively, from neutron scattering and calculations give no indication of the formation of uranyl species and the associated layered compounds as in U2O5. Surprisingly, XAFS experiments have demonstrated the opposite, showing terminal and possibly bridging U(VI)- and Np/Pu(V)-oxo moieties. This has been interpreted in U as originating in tunneling polarons. Without comprehensive neutron diffraction data from the Np and Pu oxides this same putative contradiction resulting from the differing time domains of the measurements cannot be discerned. However, we have found that Pu displays two types of H2O-induced disorder, oxidation but also simply a disruption of the Pu-O bonds that may be hydrolysis, leading to the proposal that PuO2+x is perhaps best described as PuO2+x–y(OH)2y×zH2O. In addition, the distribution of the near neighbor Pu-O bonds is affected by whether the material has been thermally treated. Disorder without oxidation has not been observed with U, with too few samples of NpO2 to know. Another significant difference among them is the absence of long range order for U4O9, where PuO2+x and NpO2+x with large x both showing the likely continuation of the fluorite structure with some additional features. Continued research on these compounds is essential for further identification and characterization of their structures and properties.

Structural chemistry of UO2, NpO2, and PuO2

253rd National Meeting (2017)

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