Analysis of the superstructure of U4O9 by neutron diffraction

Kinetics of precipitation of U4O9 from hyperstoichiometric UO2+x

“…The observed patterns also matched the I 43d structure reported by Copper and Wills [27]. Many other crystal structures reported were tried, such as U 4 O 9 space group I4 1 32 from Masaki and Doi [28], and U 4 O 9 space group I 43d from Belbboch et al [29]. Only the UO 2.234 space group I 43d structure produces the peaks around a d-spacing of 2.3 Å as seen in the diffraction patterns and thus only this crystal structure could fit the patterns for peaks generated from a U 4 O 9 (or more precisely a U 4 O 9Ày ) phase.…”

Channeling investigation of the crystalline structure ofU4O9−y

“…Long-range order techniques demonstrated that the uranium sublattice of uranium oxides in the range UO 2 -U 4 O 9−y is only slightly affected by the incorporation of extra oxygen atoms. [6][7][8][9][10][20][21][22][23][24]29,30,[65][66][67][68][69][70][71][72][73][74][75][76][77][78][79] In particular, the U 4 O 9−y oxide forms a 4 ϫ 4 ϫ 4 superstructure where U atoms are slightly displaced from their regular fluorite-type positions in a sys-tematic pattern. The face-centered-cubic structure is thus essentially preserved.…”

“…8,[20][21][22][23][24][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82] Conversely, the presence of a heterogeneous structure in the U sublattice, composed of a mixture of an intact UO 2 -like phase and of a second glassy phase recently evidenced by x-ray absorption fine-structure spectroscopy ͑XAFS͒, 26,27 has to be excluded.…”

“…64 This observation is in agreement with systematic small displacements of U atoms from fcc positions evidenced by x-ray, neutron, and electron diffraction analyses. 8,[20][21][22][23][24]29,30,[65][66][67][68][69][70][71][72][73][74][75][76][77][78][79] ͑ii͒ The channeling yield on U atoms in both investigated planes is largely enhanced with respect to UO 2 with typical values 0.40 and 0.45 for ͑001͒ and ͑110͒ planes, respectively. Thus, both monoelemental ͕100͖ U planes as well as dielemental ͕110͖ planes are distorted due to displacements of U atoms in the U 4 O 9−y crystal.…”

A conceptual model for the fuel oxidation of defective fuel

“…The best fit to the data was obtained using the UO 2 space group Fm3m from Hutchings [86] and the UO 2.234 space group I43d from Kim et al [87]. The latter phase could also be expressed as U 4 O 8.936 , (or U 4 O 9Ày , where y is 0.064), which provides further evidence that U 4 O 9 has a non-stoichiometric field at elevated temperature [88]. Although the U 4 O 9 phase is slightly hypo-stoichiometric, this observation will not significantly affect the thermodynamic analysis in Appendix A.…”