Two-and three-dimensional open-framework uranium arsenates: synthesis, structure, and characterization

Abstract

Hydrothermal reactions between uranium salts and arsenic pentoxide in the presence of two different amines yielded six new uranium arsenate phases exhibiting open-framework structures, ethylenediamine (en): [C₂N₂H₉][(UO₂)(AsO₄)], I; [C₂N₂H₁₀][(UO₂)F(HAsO₄)] ₂·4H₂O, II; [C₂N₂H₉] [U₂F₅(HAsO₄)₂], III; [C₂N₂H₉][UF₂(AsO₄)], IV; diethylenetriamine (DETA), [C₄N₃H₁₆] [U₂F₃(AsO₄)₂(HAsO₄)], V; and [C₄N₃H₁₆][U₂F₆(AsO₄) (HAsO₄)], VI. The structures were determined using single crystal studies, which revealed two- (I, II, V) and three-dimensional (III, IV, VI) structures for the uranium arsenates. The uranium atom, in these compounds, exhibits considerable variations in the coordination (6 to 9) that appears to have some correlation with the synthetic conditions. The water molecules in [C₂N₂H₁₀][(UO₂)F(HAsO₄)] ₂·4H₂O, II, could be reversibly removed, and the dehydrated phase, [C₂N₂H₁₀][(UO₂)F(HAsO₄)] ₂, IIa, was also characterized using single crystal studies. The observation of many mineralogical structures in the present compounds suggests that the hydrothermal method could successfully replicate the geothermal conditions. As part of this study, we have observed autunite, Ca[(UO₂)(PO₄)]₂(H₂O)₁₁, metavauxite, [Fe(H₂O)₆][Al(OH)(H₂O)(PO₄)]₂, linarite, PbCu(SO₄)(OH)₂, and tancoite, LiNa₂H[Al(PO₄)₂(OH)], structures. The repeated observation of the secondary building unit, SBU-4, in many of the uranium arsenate structures suggests that these are viable building units. Optical studies on the uranium arsenate compound, [C₄N₃H₁₆][U₂F₆ (AsO₄)(HAsO₄)], VI, containing uranium in the +4 oxidation state indicates a blue emission through an upconversion process. The compound also exhibits antiferromagnetic behavior.