Trivalent Cation-Controlled Phase Space of New U(IV) Fluorides, Na3MU6F30 (M = Al3+, Ga3+, Ti3+, V3+, Cr3+, Fe3+): Mild Hydrothermal Synthesis Including an in Situ Reduction Step, Structures, Optical, and Magnetic Properties

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A series of new, complex U(IV) fluorides, namely, Na3MU6F30 (M = Al3+, Ga3+, Ti3+, V3+, Cr3+, and Fe3+), containing trivalent transition- and main-group metal cations were synthesized via an in situ reduction step of U(VI) to U(IV). Single crystals of the series were grown in high yield under mild hydrothermal conditions and were characterized by single-crystal X-ray diffraction. The reported compounds crystallize in the trigonal space group Pc1 and exhibit complex crystal structures with a three-dimensional (3-D) framework composed of corner- and edge-shared UF9 polyhedra. The arrangement of U2F16 dimers forms two types of hexagonal channels, where MF6 polyhedra and sodium atoms are located. The most interesting structural feature is the presence of the 3-D framework that can accommodate various transition-metal ions in low oxidation states, indicating that the framework acts as an excellent host. Trivalent transition metal ions, even reduced Ti3+and V3+, were stabilized by both the rigid framework and by our synthetic conditions. Utilizing ionic radii of transition metal ions, a phase boundary was investigated, suggesting that there exists a size limit for the M site in the crystal structure. The valence state of uranium was studied by U 4f X-ray photoelectron spectroscopy, which confirmed the presence of U4+. Temperature-dependent magnetic susceptibility measurements yielded effective magnetic moments of 3.50 and 3.35 μB for Na3MU6F30 (M = Al3+ and Ga3+), respectively. For the other compounds, combined effective magnetic moments of 8.93, 9.09, 9.18, and 10.39 μB were obtained for Ti, V, Cr, and Fe members, respectively. In all cases, large negative Weiss constants were observed, which are indicative of the existence of a spin gap in U4+. Field-dependent magnetic property measurements at 2 K for Na3FeU6F30 demonstrated that U4+ attains a nonmagnetic singlet ground state at low temperature. Optical and thermal properties were measured and are reported.