Actinide Sulfate Structures from Caustic Solvents

Four unique actinide sulfates were synthesized using solvothermal techniques with strong acids. The first plutonium(III) sulfate structure, Pu(HSO₄)₃, was synthesized and is isostructural with analogous lanthanide-based frameworks. A similar synthesis approach yielded crystals of NpNa_0.5(HSO₄)₁₅(SO₄)_1.5, which has a comparable framework to the Pu(III) compound, but the neptunium metal is tetravalent and sodium is incorporated into the structure, as confirmed by chemical analysis. Anhydrous neptunium sulfate, Np(SO₄)₂, is reported and is isotypic with U(SO₄)₂. Finally, (H₃O)₂(UO₂)(SO₄)₂, which contains a uranyl sulfate sheet structure, was synthesized and characterized. The corresponding sheet anion topology has previously been reported with various oxyanions, but this is the first report that contains sulfate. The sheets are charge balanced by hydronium cations in the interstitial space. This compound readily degrades and forms crystals of the synthetic analogue to the uranium mineral shumwayite, which is likely thermodynamically favorable. All four of these actinide sulfate compounds were synthesized in extremely acidic media, resulting in interesting and unique structures.