New Structural Insights and High Temperature Phase Transitions in Bismuth and Lead Uranates

Despite extensive implementation of uranium and its oxides throughout the nuclear fuel cycle, studies of the ternary uranate structures are limited. The Bi-U-O and Pb-U-O systems are at the forefront of recent scientific development in the context of the lead bismuth eutectic cooled fast reactors. Given the social importance and the gap in our understanding of the solid state and crystal chemistry of uranium oxides, and the development of modern robust crystallographic techniques, there is compelling reasoning to return to studying uranium oxides.Bi2UO6 is one of few known structures in the Bi-U-O system. Our investigation of the thermal response of Bi2UO6 has demonstrated the inclusion of superlattice reflections not previously reported [1], which we have linked to the formation of oxygen vacancies. Using high resolution synchrotron X-ray powder diffraction and Neutron Powder Diffraction (NPD) in combination with DFT studies from first principles, we have resolved a new cell and accurate structure for Bi2UO6, crystallising in space group P-3m1. PbUO4 is a well-known member of the Pb-U-O system with orthorhombic Pbcm symmetry as determined by single crystal X-ray diffraction.[2] Previous studies of the thermal response of PbUO4 conditions have revealed little structural changes [3], however our investigations have demonstrated the irreversible transformation of PbUO4 to an unreported fluorite-related phase, nominally ‘Pb2U2O7’. Using a combination of NPD and DFT calculations, we have resolved the structure of Pb2U2O7 as the orthorhombic pyrochlore in space group Imma.These findings contribute toward bridging the gap of understanding regarding the structures and response to external stimuli of some of the simplest uranium oxides. Establishing such knowledge of fundamental uranium chemistry has implications for both the nuclear and crystallographic fields. With further adoption of nuclear energy, there is a growing need for a better understanding in uranium chemistry to facilitate the safe handling and storage of spent nuclear fuels and potential alteration phases. As it follows, establishing a strong understanding of uranium crystal chemistry, exhibiting 5f electron chemistry, may provide unique insights relevant to functional material design, as is demonstrated by the interesting capabilities of thermally induced oxygen defect ordering not previously realised.[1] Koster, A.; Renaud, J.; Rieck, G. The crystal structures at 20 and 1000° C of bismuth uranate, Bi2UO6. Structural Science 1975, 31 (1), 127-131.[2] Cremers, T.; Eller, P.; Larson, E.; Rosenzweig, A. Single-crystal structure of lead uranate (VI). Crystal Structure Communications 1986, 42 (12), 1684-1685.[3] Popa, K.; Beneš, O.; Staicu, D.; Griveau, J.-C.; Colineau, E.; Raison, P.; Vigier, J.-F.; Pagliosa, G.; Sierig, M.; Vălu, O. Thermal properties of PbUO4 and Pb3UO6. Journal of Nuclear Materials 2016, 479, 189-194