Synthetic Strategies for the Synthesis of Ternary Uranium(IV) and Thorium(IV) Fluorides

A series of new U(IV) and Th(IV) fluorides, Na7U6F31 (1), NaUF5 (2), NaU2F9 (3), KTh2F9 (4), NaTh2F9 (5), (H3O)Th3F13 (6), and (H3O)U3F13 (7), was obtained using hydrothermal and low-temperature flux methods. Mild hydrothermal reactions with uranyl acetate as a precursor yielded 1, 7, and the monoclinic polymorph of NaU2F9, whereas direct reactions between UF4 and NaF led to the formation of 2 and orthorhombic NaU2F9 (3). This highlights an unexpected difference in reaction products when different starting uranium sources are used. All seven compounds were characterized by single-crystal X-ray diffraction, and their structures are compared on the basis of cation topology, revealing a close topological resemblance between fluorides on the basis of the layers observed in NaUF5(H2O). Phase-pure samples of 1, 2, and both polymorphs of NaU2F9 were obtained, and their spectroscopic and magnetic properties were measured. The UV–vis data are dominated by the presence of U4+ cations and agree well with the electronic transitions. Effective magnetic moments of the studied compounds were found to range from 3.08 to 3.59 μB.