Application of a Mild Hydrothermal Approach Containing an in Situ Reduction Step to the Growth of Single Crystals of the Quaternary U(IV)-Containing Fluorides Na4MU6F30 (M = Mn2+, Co2+, Ni2+, Cu2+, and Zn2+) Crystal Growth, Structures, and Magnetic Properties

A family of rare U(IV)-containing quaternary fluorides, Na₄MU₆F₃₀ (M = Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺), was synthesized in single crystal form via a mild hydrothermal technique utilizing an in situ U(VI) to U(IV) reduction step. The modified hydrothermal route is described, and the conditions to obtain single crystals in high yield are detailed. The crystal structures were determined by single crystal X-ray diffraction. The isostructural fluorides crystallize in a new structure type in the trigonal space group P3̅c1. They exhibit a complex three-dimensional crystal structure consisting of corner- and edge-shared UF₉ and MF₆ polyhedra. The main building block, a U₆F₃₀⁶– group, is arranged to create two distinct hexagonal channels, inside which MF₆ octahedra and Na⁺ cations are located. The copper-containing member of the series, Na₄CuU₆F₃₀, is unusual in that the Cu²⁺ cation exhibits a rare symmetrical coordination environment consisting of six identical Cu–F bond distances, indicating the lack of the expected Jahn–Teller distortion. Magnetic susceptibility measurements of Na₄ZnU₆F₃₀ yielded an effective magnetic moment of 3.42 μB for the U⁴⁺ (f²) cation in the structure. Measurements of the other members containing magnetic transition-metal cations in addition to U⁴⁺, Na₄MU₆F₃₀ (M = Mn²⁺, Co²⁺, Ni²⁺, and Cu²⁺) yielded total effective magnetic moments of 10.2, 9.84, 8.87, and 8.52 μB for the Mn-, Co-, Ni-, and Cu-containing materials, respectively. No evidence for long-range magnetic ordering was found down to 2 K. Measurements of the magnetization as a function of applied magnetic field at 2 K for Na₄MnU₆F₃₀ confirmed that the U⁴⁺ magnetic cation exhibits a nonmagnetic singlet ground state at low temperature. Thermal stability measurements and UV–vis diffuse reflectance spectroscopy are also reported.