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 <i>P</i>3̅<i>c</i>1. They exhibit a complex three-dimensional crystal structure consisting of corner- and edge-shared UF₉ and MF₆ polyhedra. The main building block, a U₆F₃₀^6– 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, Na₄CuU₆F_30, 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 μ_B 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 μ_B 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