Electronic structure and O K-edge XAS spectroscopy of U3O8

Understanding the uranium 5f/6d orbital mixing with oxygen 2p valence orbitals in uranium oxides is important for advancing nuclear technology. Unfortunately, U-O orbital mixing is difficult to probe experimentally. In this manuscript, U-O bonding is evaluated in U3O8 using O K-edge X-ray absorption spectroscopy (XAS). To confirm that the O K-edge XAS spectra were correct and did not contain contributions from surface contamination, three different sample types were investigated using three unique detection methods. Specifically an epitaxial film of U3O8 deposited on Al2O3 (PAD-U3O8) was probed using grazing-incidence fluorescence yield (GIFY) detection, a bulk powder of α-phase U3O8 was analyzed with fluorescence yield (FY) detection at normal incidence, and particles of α-phase U3O8 were studied in transmission mode using a scanning transmission X-ray microscope (STXM). Experimental spectra have been presented in the context of previously published computational results from DFT using the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional. Overall, the comparative analyses of PAD-U3O8 and α-phase U3O8 samples enabled identification of unique signatures associated with oxygen 2p orbital mixing with both UV and UVI 5f and 6d valence orbitals.