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

Polycrystalline U₃O₇ powder was synthesized by oxidation of UO₂ powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the U₃O₇ 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 U₄O_9-ε (more precisely U₆₄O₁₄₃), U₃O₇ exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO₂. 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 (U₁₅O₃₅), and spanned by basis vectors A = a_p - 2b_p, B = -2a_p + b_p, and C = 3c_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 U₃O₇ results in a layered structure, which is different from the well-known U₄O_9-ε crystal structure.