9-4 Probing the Water Environment of Uranium and Thorium Compounds with Neutron Diffraction and Inelastic Neutron Scattering

Session: Early Career Investigators in Mineralogy and Crystallography

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When using X-ray diffraction to solve the crystal structures of materials containing heavy atoms, such as uranium minerals or compounds, the high electron density of the heavy atom can often preclude the resolution of the light atoms, such as hydrogen, in the structure. In this case, neutron scattering techniques, including Neutron diffraction (ND) and inelastic neutron scattering (INS), offer much greater sensitivity to hydrogen atoms than methods such as X-ray and electron diffraction. To this end, we use infrared spectroscopy, neutron diffraction, and inelastic neutron scattering to probe the water environments of hydrated and deuterated uranium and thorium oxalate compounds: An(IV)(C₂O₄)₂ ∙ xH₂O and An(IV)(C₂O₄)₂ ∙ xD₂O (An = Th, U; x = 0-2, 6). These compounds, along with their Np and Pu structural analogues, are constructed of layers of actinide oxalate sheets with water occupying the interlayer space. Despite the relevance of these phases to the nuclear fuel cycle and similarities to mineral phases, their crystal structures were only published in 2023, and the high electron density of the actinide atoms coupled with the disorder of several water molecules prevented locating the hydrogen atoms in the structures. Through powder ND studies at the High Flux Isotope Reactor and INS studies at the Spallation Neutron Source, both at Oak Ridge National Laboratory, we can better understand the roles water molecules play in the structures of these actinide phases.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9412

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