Optical vibrational spectra and proposed crystal structure of ε-UO₃

ε-UO₃ is an exotic polymorph in the uranium trioxide system with an undetermined crystal structure and limited optical vibrational spectroscopic data. To improve understanding of this compound, we synthesize and investigate the crystal structure and optical vibrational spectra of ε-UO₃. Infrared spectra collected for ε-UO₃ are in good agreement with previously published results, and our studies extend the available data into the low-energy (600–100 cm^–1) regime. For the first time, Raman spectra are presented for ε-UO₃ using both 785 and 532 nm excitation wavelengths. Previous reports suggest an impurity phase may be present in ε-UO₃ produced by calcination of U₃O₈; however, spectral center-of-mass calculations, principal component analyses, and Raman spectroscopic mapping employed to investigate this possibility indicate that the product of U₃O₈ calcined in O₃(g) in this work is likely phase-pure. A possible novel structure solution for ε-UO₃ is determined via Rietveld refinement of powder X-ray diffraction data and is triclinic, P-1, with a = 4.01 Å, b = 3.85 Å, c = 4.18 Å, and α = 98.26°, β = 90.41°, γ = 120.46° (R_wp = 8.30%). The asymmetric unit of ε-UO₃ consists of U(VI) in hexagonal bipyramidal coordination with displaced equatorial oxygen. Further analysis reveals that the structure of ε-UO₃ is best described by a 2 × 1 × 2 supercell structure in P-1 with a = 8.03 Å, b = 3.86 Å, c = 8.37 Å with α = 98.26°, β = 90.41°, and γ = 120.46°, although a higher-symmetry structure is possible. Optical vibrational spectroscopic and structural measurements of ε-UO₃ presented here furthers our understanding of this complex uranium oxide and clarifies the origin of reported structural similarity to U₃O₈.