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

Polycrystalline UO powder was synthesized by oxidation of UO powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the UO 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 UO (more precisely UO), UO exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO. 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 (UO), and spanned by basis vectors A = a - 2b, B = -2a + b, and C = 3c (lattice parameters of the subcell are a = b = 538.00 ± 0.02 pm and c = 554.90 ± 0.02 pm; c/a = 1.031). The arrangement of cuboctahedra in UO results in a layered structure, which is different from the well-known UO crystal structure.