Optical vibrational spectra and proposed crystal structure of ε-UO₃

Abstract e-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 e-UO3. Infrared spectra collected for e-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 e-UO3 using both 785 and 532 nm excitation wavelengths. Previous reports suggest an impurity phase may be present in e-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 e-UO3 is determined via Rietveld refinement of powder X-ray diffraction data and is triclinic, P-1, with a = 4.01 A, b = 3.85 A, c = 4.18 A, and α = 98.26 °, β = 90.41 °, γ = 120.46 ° (Rwp = 8.30 %). The asymmetric unit of e-UO3 consists of U(VI) in hexagonal bipyramidal coordination with displaced equatorial oxygen. Further analysis reveals that e-UO3 is best described by a 2 × 1 × 2 supercell structure in P-1 with a = 8.03 A, b = 3.86 A, c = 8.37 A 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 e-UO3 presented here furthers our understanding of this complex uranium oxide and clarifies the origin of reported structural similarity to U3O8.