A New Look at the Structural Properties of Trisodium
Uranate Na<sub>3</sub>UO<sub>4</sub>
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Abstract
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 <sup>23</sup>Na multiquantum magic angle
spinning nuclear magnetic resonance. The compound, isostructural with
Na<sub>3</sub>BiO<sub>4</sub>, 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-Na<sub>3</sub>(U<sub>1–<i>x</i></sub>,Na<sub><i>x</i></sub>)O<sub>4</sub> (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-Na<sub>3</sub>(U<sub>1–<i>x</i></sub>,Na<sub><i>x</i></sub>)O<sub>4</sub>, 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>