Structural
Investigation of Uranium–Neptunium
Mixed Oxides Using XRD, XANES, and <sup>17</sup>O MAS NMR
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Abstract
Uranium–neptunium mixed dioxides
are considered as fuels
and targets for the transmutation of the minor actinides in fast neutron
reactors. Hereafter, a local and atomic scale structural analysis
was performed on a series of U<sub>1–<i>x</i></sub>Np<sub><i>x</i></sub>O<sub>2</sub> (<i>x</i> =
0.01; 0.05; 0.20; 0.50; 0.75; 0.85) synthesized by the sol–gel
external gelation method, for which longer range structural analysis
indicates that the process yields solid solutions. The oxidation state
of IV for uranium and neptunium cations was confirmed using U L<sub>III</sub> and Np L<sub>III</sub> edge X-ray absorption near edge
structure (XANES). The atomic scale structure was probed with <sup>17</sup>O magic angle spinning nuclear magnetic resonance (MAS NMR)
for the anion. Structural distortions due to the substitution of U
by the smaller Np cation were detected by <sup>17</sup>O MAS NMR