Structural Investigation of Uranium–Neptunium Mixed Oxides Using XRD, XANES, and ¹⁷O MAS NMR

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_1–<i>x</i>Np_<i>x</i>O₂ (<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_III and Np L_III edge X-ray absorption near edge structure (XANES). The atomic scale structure was probed with ¹⁷O 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 ¹⁷O MAS NMR

High-Resolution Solid-State Oxygen-17 NMR of Actinide-Bearing Compounds: An Insight into the 5f Chemistry

A massive interest has been generated lately by the improvement of solid-state magic-angle spinning (MAS) NMR methods for the study of a broad range of paramagnetic organic and inorganic materials. The open-shell cations at the origin of this paramagnetism can be metals, transition metals, or rare-earth elements. Actinide-bearing compounds and their 5f unpaired electrons remain elusive in this intensive research area due to their well-known high radiotoxicity. A dedicated effort enabling the handling of these highly radioactive materials now allows their analysis using high-resolution MAS NMR (>55 kHz). Here, the study of the local structure of a series of actinide dioxides, namely, ThO₂, UO₂, NpO₂, PuO₂, and AmO₂, using solid-state ¹⁷O MAS NMR is reported. An important increase of the spectral resolution is found due to the removal of the dipolar broadening proving the efficiency of this technique for structural analysis. The NMR parameters in these systems with numerous and unpaired 5f electrons were interpreted using an empirical approach. Single-ion model calculations were performed for the first time to determine the <i>z</i> component of electron spin on each of the actinide atoms, which is proportional to the shifts. A similar variation thereof was observed only for the heavier actinides of this study

A New Look at the Structural Properties of Trisodium Uranate Na₃UO₄

The crystal structure of trisodium uranate, which forms following the interaction between sodium and hyperstoichiometric urania, has been solved for the first time using powder X-ray and neutron diffraction, X-ray absorption near-edge structure spectroscopy, and solid-state ²³Na multiquantum magic angle spinning nuclear magnetic resonance. The compound, isostructural with Na₃BiO₄, has monoclinic symmetry, in space group <i>P</i>2/<i>c</i>. Moreover, it has been shown that this structure can accommodate some cationic disorder, with up to 16(2)% sodium on the uranium site, corresponding to the composition α-Na₃(U_1–<i>x</i>,Na_<i>x</i>)­O₄ (0 < <i>x</i> < 0.18). The α phase adopts a mixed valence state with the presence of U­(V) and U­(VI). The two polymorphs of this compound described in the literature, <i>m</i>- and β-Na₃(U_1–<i>x</i>,Na_<i>x</i>)­O₄, have also been investigated, and their relationship to the α phase has been established. The completely disordered low-temperature cubic phase corresponds to a metastable phase. The semiordered high-temperature β phase is cubic, in space group <i>Fd</i>3̅<i>m</i>