Assessment of the U3O7 Crystal Structure by X-ray and Electron Diffraction

Polycrystalline U 3 O 7 powder was synthesized by oxidation of UO 2 powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the U 3 O 7 phase was measured as 2.34 ± 0.01. The crystal structure was assessed from X-ray diffraction (XRD) and selected-area electron diffraction (SAED) data. Similar to U 4 O 9−ε (more precisely U 64 O 143 ), U 3 O 7 exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO 2 . Cations remain arranged identical to that in the fluorite structure, and excess anions form distorted cuboctahedral oxygen clusters, which periodically replace the fluorite anion arrangement. The structure can be described in an expanded unit cell containing 15 fluorite-like subcells (U 15 O 35 ), and spanned by basis vectors A = a p – 2 b p, B = −2 a p + b p, and C = 3 c p (lattice parameters of the subcell are a p = b p = 538.00 ± 0.02 pm and c p = 554.90 ± 0.02 pm; c p / a p = 1.031). The arrangement of cuboctahedra in U 3 O 7 results in a layered structure, which is different from the well-known U 4 O 9−ε crystal structure.