Molten Salt Synthesis of Uranium Monocarbide Below 1000 °C.

We report a versatile molten salt synthesis (MSS) route to uranium monocarbide (UC) that proceeds below 1000 °C and achieves complete conversion within 1 h. The method employs lithium hydride (LiH) as a salt-soluble reductant/oxygen scavenger and lithium chloride (LiCl) as the molten medium, enabling straightforward product isolation by salt dissolution in polar aprotic solvents. The process tolerates diverse uranium feedstocks, including UO2, UO3, UCl4, and UBr5, yielding phase-pure UC without coformation of higher-order carbides under the conditions studied herein. Powder X-ray diffraction (XRD) confirms the NaCl-type structure of UC, with a-axis contraction correlated to nonstoichiometric UO defects within the UC lattice, consistent with empirical literature data. Scanning electron microscopy (SEM) reveals larger particles (300-500 nm) for oxide-derived UC and finer particles (100-200 nm) for halide-derived UC, consistent with differences in precursor solubility and crystallization kinetics. This low-temperature, rapid, and feedstock-flexible route addresses historical challenges associated with carbothermal synthesis and supports future fabrication of UC-based fuels, including next-generation transuranic (TRU) fuel fabrication.