Mild Hydrothermal Crystal Growth, Structure, and Magnetic Properties of Ternary U(IV) Containing Fluorides: LiUF₅, KU₂F₉, K₇U₆F₃₁, RbUF₅, RbU₂F₉, and RbU₃F₁₃

Single crystals of several ternary alkali uranium fluorides, LiUF₅, KU₂F₉, K₇U₆F₃₁, RbUF₅, RbU₂F₉, and RbU₃F₁₃, have been obtained in a mild hydrothermal process using UO₂(CH₃CO₂)₂(H₂O)₂ as the uranium source. Their crystal structures were determined by single crystal X-ray diffraction. The uranium in the starting reagent was successfully reduced from U⁶⁺ to U⁴⁺ in a dilute hydrofluoric acid environment, aided by the presence of a copper salt. All materials exhibit highly complex crystal structures that range from two-dimensional to three-dimensional. The U⁴⁺ cations are found in high (UF₈ and UF₉) coordination environments. The magnetic susceptibility measurements yielded effective magnetic moments of 3.01–3.83 μ_B for the U⁴⁺ cations. The temperature dependent magnetic susceptibility measurements confirmed that the U⁴⁺ cation exhibits a nonmagnetic singlet ground state at low temperatures. No long-range magnetic order was observed for any of the above compositions down to 2 K. Optical and thermal behaviors of the fluorides were also investigated.