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 hyper-stoichiometric 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 multi quantum magic angle spinning nuclear magnetic resonance. The compound, isostructural with Na₃BiO₄, has monoclinic symmetry, in space group P2/c. 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-x,Na_x)O₄ (0 ≤ x ≤ 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, m- and β- Na₃(U_1-x,Na_x)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 semi ordered high-temperature β phase is cubic, in space group Fd3(-bar)m. (authors)