The unusual structural chemistry of uranium: controlling phase transformations in ternary uranium oxides

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Society of Crystallographers in Australia and New Zealand
Structural investigations of ternary uranium oxides are pertinent to the management of spent nuclear fuel since such environmentally hazardous and potentially dangerous secondary phases may form during the process and/or storage of the material [1]. They further allow for the exploration of the peculiar, exotic and poorly known properties of materials containing, or which can access 5f electrons. The rhombohedral oxides AUO4 for A = α-Sr or Ca in space group 𝑅𝑅3�𝑚𝑚 exemplifies this. We have found through a combination of in situ synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, that α-SrUO4 undergoes a fascinating first-order phase transformation under oxidising conditions [2]. This involves the synergetic loss of lattice oxygen resulting in oxygen vacancy defect formation and reduction of the uranium cations, which seemingly reduces the activation energy barrier for transformation to its orthorhombic form, β-SrUO4. Under similar conditions, CaUO4 does not display any transformative behaviour, however, defects can be engineered through the substitution of Ca2+ for Sr2+ in the solid solution α-SrxCa1-xUO4 when heated to high temperature under oxidising conditions. The introduction of Sr2+ cations in α-SrxCa1-xUO4 was found to decrease the temperature at which oxygen vacancy defects form. This phenomenon was rationalised as a consequence of the introduction of Sr2+ cations leading to lattice expansion, which causes the proximity of defects to increase. This subsequently reduces free energy increasing defect-defect interactions, allowing defects to form at lower temperature. Remarkably, when heated under reducing conditions, the disordered oxygen defect containing rhombohedral α-SrUO4-x structure undergoes a reversible first-order phase transformation that involves the ordering of the oxygen defects resulting in lowering of the crystallographic symmetry to triclinic in space group P 1� denoted δ-SrUO4-x. This remarkable transformation, which implies entropy is being decreased as temperature increases, could be replicated in CaUO4 and also in α-Sr0.4Ca0.6UO4 where the transformation temperature is reduced by increasing the Sr2+ content, consistent with the effects of reducing defect-defect interactions. The S-XRD data shows the structure of δ-CaUO4-x to be incommensurate whereas δ-SrUO4-x is commensurate. This implies a miscibility gap may exist between the isostructural CaUO4 and α-SrUO4 related to shortrange order. This investigation demonstrates the rich and fascinating crystal chemistry present in uranium oxides, which, in some cases, may have profound societal importance if it can either be safely used or if associated properties can be replicated into non-actinide materials.
Uranium oxides, X-ray diffraction, Orthorhombic lattices, Electrons, Spent fuels, Oxygen, Vacancies, Phase transformations
Murphy, G. L., Wang, C.-H., Beridze, G., Zhanging, Z., Avdeev, M., Kowalski, P. M., Brand, H., Johannessen, B., & Kennedy, B. (2017). The unusual structural chemistry of uranium: controlling phase transformations in ternary uranium oxides. Paper presented at CRYSTAL 31, the 31st Biennial Conference of the Society of Crystallographers in Australia and New Zealand, Pullman Bunker Bay, Western Australia, 3 – 7 December 2017. Retrieved from: