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 NpNa0.5(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.

Pu(VI) Oxalate Crystal Structure and Evidence of Photoreduction to Pu(IV) Oxalate.

We report the first crystal structure of a Pu(VI)-oxalate compound. This compound, [PuO2(C2O4)(H2O)]·2(H2O) (1), crystallizes in space group P21/c with a = 5.5993(3) Å, b = 16.8797(12) Å, c = 9.3886(6) Å, and ß = 98.713(6)°. It is isostructural with the previously reported U(VI) compound, [UO2(C2O4)(H2O)]·2(H2O). Each plutonyl ion (PuO22+) is coordinated in the equatorial plane by two side-on bidentate oxalates, creating an infinite chain along [001]. A coordinated water molecule and twisting of the oxalates lead to a distorted pentagonal bipyramidal geometry of the Pu. A photochemical degradation was observed for 1, which resulted in the formation of a secondary crystalline phase. The absorption spectrum of this secondary phase confirmed the presence of Pu(IV), but it did not match the spectrum of Pu(C2O4)2·6H2O, which is considered to be the primary product of Pu-oxalate precipitation. While compound 1 has previously been proposed to exist in solution, this is the first time it has been isolated via crystallization. Although redox interactions between Pu and oxalate have been documented in the literature, the present study is the first observation of a photochemical reduction of Pu(VI)-oxalate. As a result, this study has expanded on the limited understanding of the Pu(VI)-oxalate system, which is important for nuclear fuel cycle applications.

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries.

Invited for the cover of this issue is the group of Amy Hixon at the University of Notre Dame. The image depicts the newly identified structure of a PuIV oxalate sheet compared to the historically assumed structure. Read the full text of the article at 10.1002/chem.202301164.

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries.

Plutonium(IV) oxalate hexahydrate (Pu(C2 O4 )2 ⋅ 6 H2 O; PuOx) is an important intermediate in the recovery of plutonium from used nuclear fuel. Its formation by precipitation is well studied, yet its crystal structure remains unknown. Instead, the crystal structure of PuOx is assumed to be isostructural with neptunium(IV) oxalate hexahydrate (Np(C2 O4 )2 ⋅ 6 H2 O; NpOx) and uranium(IV) oxalate hexahydrate (U(C2 O4 )2 ⋅ 6 H2 O; UOx) despite the high degree of unresolved disorder that exists when determining water positions in the crystal structures of the latter two compounds. Such assumptions regarding the isostructural behavior of the actinide elements have been used to predict the structure of PuOx for use in a wide range of studies. Herein, we report the first crystal structures for PuOx and Th(C2 O4 )2 ⋅ 6 H2 O (ThOx). These data, along with new characterization of UOx and NpOx, have resulted in the full determination of the structures and resolution of the disorder around the water molecules. Specifically, we have identified the coordination of two water molecules with each metal center, which necessitates a change in oxalate coordination mode from axial to equatorial that has not been reported in the literature. The results of this work exemplify the need to revisit previous assumptions regarding fundamental actinide chemistry, which are heavily relied upon within the current nuclear field.

Targeting Diverse Bridging Motifs within Actinide Borosulfates and Establishing an Unconventional Structural Hierarchy.

The first actinide borosulfates, (UO2)[B(SO4)2(SO3OH)] (TSUBOS-1) and (UO2)2[B2O(SO4)3(SO3OH)2] (TSUBOB-1), were synthesized solvothermally in oleum using UO3. The classical borosulfate crystal structure of TSUBOS-1 is partially consistent with an established conventional hierarchy. Uranyl pentagonal bipyramids limit the anionic network linkages and isolate sulfate tetrahedra within the anionic network. Therefore, the classical one-dimensional chain established in the hierarchy does not fully describe the structure. The structure of TSUBOB-1 is the first actinide borosulfate that contains an unconventional borate-to-borate bridging mode (denoted B-O-B) and a zero-dimensional oxoanionic unit consisting of one sulfate tetrahedron that shares vertices with two B-O-B bridged borate tetrahedra that each share a vertex with two sulfate tetrahedra. As this structure departs from the existing structural hierarchy, a modified approach for understanding the unconventional borosulfate substructure and dimensionality is proposed, together with a new graphical notation. In the course of our synthesis experiments, a novel uranyl disulfate compound (UO2)2[(S2O7)(SO3OH)2] (TSUDS) was isolated and characterized. The structure of TSUDS is a framework consisting of uranyl pentagonal bipyramids and sulfate tetrahedra. Each uranyl pentagonal bipyramid is surrounded by five sulfate tetrahedra, two of which share a vertex creating a disulfate with a S-O-S bridging mode. The uranyl bipyramids are linked to one another via the singular sulfate or disulfate groups.