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

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 P3̅c1 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 μ_B 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 μ_B 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.