Tailoring Triuranium Octoxide into Multidimensional Uranyl Fluoride Micromaterials

Uranium microstructured materials with controlled size and shape are relevant to the nuclear industry and have found applications as targets for medical isotope production, fuels for nuclear reactors, standards for nuclear forensics, and energy sources for space exploration. Until now, most studies at the microscale have focused on uranium microspheres (oxides, nitrides, carbides, and fluorides), while micromaterials of uranium halides, carbides, and pnictides with other morphologies are largely unknown. A promising method to shape the morphology of uranium micromaterials is the replacement of O by F atoms in oxide materials using a solid-gas reaction. Here, with the aim to elaborate unexplored uranium fluoride micromaterials, the fluorination of uranium oxide (U₃O₈ and UO₂) microspheres (ms), microrods (mr), and microplates (mp) in an autoclave at 250 °C with HF_(g) (produced from the thermal decomposition of silver bifluoride (SBF)) and with ammonium bifluoride (ABF) was evaluated. We show that the reactions between U₃O₈ mr and U₃O₈ mp and SBF provided the most efficient way to elaborate mr and mp UO₂F₂ micromaterials in a high yield (∼90%). The resulting UO₂F₂ mr (length: 3-20 μm) and UO₂F₂ mp (width: 1-7.5 μm) exhibited a well-defined geometry that was identical to that of the U₃O₈ precursors. Agglomerated (NH₄)₃UO₂F₅ and UO₂F₂ ms (2-3.5 μm) were prepared from the reaction of U₃O₈ ms with ABF. It is noted that the reaction of UO₂ ms with SBF and ABF did not provide any uranium fluoride micromaterials. The successful preparation of uranium fluoride microstructures (ms, mr, and mp) developed here opens the way to novel actinide fluoride micromaterials.