Investigation of uranium oxide hydrates with barium(II) ions: structural diversity, uranium valences and implications

Although barium(II) ions play an important role in the natural weathering of the mineral uraninite (UO2+x) and are expected to therefore do the same for spent nuclear fuel in geological disposal, the detailed structural understanding of this role is rather limited. In this work, we report the synthesis and characterisation of two new uranium oxide hydrate (UOH) phases with barium(II) ions, a layer-structured UOH-Ba1 constructed with undulating uranyl oxide hydroxide layers incorporating hydrated interlayer barium(II) ions; and UOH-Ba2 forming a complex three-dimensional structure by linking six different types of uranium polyhedrons with barium(II) ions located in the narrow channels. Both crystal structures were revealed using synchrotron single crystal X-ray diffraction and confirmed with powder X-ray or electron diffractions. While vibrational modes were elucidated by Raman, electronic structures and the presence of pentavalent uranium were investigated using diffuse reflectance spectroscopy. This work has provided insights into the complexity of uranium crystal chemistry in the presence of barium(II) ions and has implications to both uranium geochemistry in terms of the natural weathering of uraninite and the disposal of spent nuclear fuel in the underground repository.