Optical vibrational spectra and proposed crystal structure of ε-UO3

Elucidating the Composition and Structure of Uranium Oxide Powders Produced via NO2 Voloxidation

“…The first sample from the second oxidation step contains bright orange cryptocrystalline powder, as well as additional black particles (Figure S2). The appearance of this sample, particularly the mixture of orange and black particles, is consistent with observations from Spano et al for ε-UO 3 …”

“…This suggests that the kinetics of the first oxidation may be more rapid than expected. However, the first two oxidation steps produce primarily UO 3 , which is in agreement with the expected reaction mechanisms and the literature evidence of UO 2 oxidation in NO 2 (g). , The formation of this specific polymorph when U 3 O 8 is exposed to NO 2 (g) has been previously reported, , but the production of ε-UO 3 , specifically the ε polymorph of UO 3 , is notable, suggesting that a U 3 O 8 precursor exists during oxidation, because all reported production methods for ε-UO 3 require U 3 O 8 and a pseudomorphic decomposition reaction from U 3 O 8 to ε-UO 3 , which has been suggested in previous works. ,, Furthermore, the presence of this phase, with only minor U 3 O 8 contributions, supports the posed hypothesis of rapid kinetics during the initial oxidation step. The microstructural features seen during the first two oxidation steps, particularly cracking in the surface of the uranium oxide phases, suggest that the gases present in the reaction may be acting within grain boundaries or other pre-existing surface anomalies in the ceramic.…”

“…Information on solid phases from voloxidation in the literature rely on color of the material , as a characterization technique and diagnostic method for the solid phase. Recently, Spano et al identified that for ε-UO 3 , multiple colors can coexist within a single sample, but despite that, crystalline phases and optical vibrational properties are identical . This finding implies that color is not an appropriate characterization technique for uranium oxide materials and underscores that thorough characterization of uranium solids involved in these reactions is imperative.…”

Unexpected features in the optical vibrational spectra of δ-UO3

“…The diverse crystal chemistry and resulting variety of optical vibrational (Raman and IR) spectroscopic features of members of the uranium trioxide polymorph system have been of interest to researchers for several decades (Sheft et al, 1950;Hoekstra and Siegel, 1961;Cornman, 1962;Sweet et al, 2013). UO 3 is an important intermediate in U processing in both front (Cornman, 1962) and proposed back-end (Johnson et al, 2017;Spano et al, 2021) fuel cycle processes. Resultingly, some polymorphs within UO 3 phase space, such as the α, γ, and amorphous modifications are easily synthesized with extensive literature reports of these materials (Loopstra and Cordfunke, 1966;Sweet et al, 2013;Guo et al, 2016;Colmenero et al, 2017).…”

“…Resultingly, some polymorphs within UO 3 phase space, such as the α, γ, and amorphous modifications are easily synthesized with extensive literature reports of these materials (Loopstra and Cordfunke, 1966;Sweet et al, 2013;Guo et al, 2016;Colmenero et al, 2017). Other, more exotic polymorphs such as βand ε-UO 3 have only recently been investigated for their structural and optical spectroscopic properties (Spano et al, 2020;Spano et al, 2021).…”