Fingerprinting Uranium Oxides with Electron Energy Loss Spectroscopy Supported by Theoretical Computations

Uranium
oxides occur in a variety of phases that differ
in their
crystal structure and uranium oxidation states. Electron energy loss
spectroscopy (EELS) is one of the few techniques that has sufficient
spatial resolution and sensitivity to electronic structure to distinguish
among phases at the nanoscale. However, beam-sensitive materials,
such as uranium oxides, are subject to spectral modification due to
interactions with the electron beam. Therefore, theory support is
essential to reliably exclude the impact of beam damage and generate
true reference data sets. Here, we use a comparison of theoretical
and experimental spectra to probe the impact of beam damage on the
O K-edge and U N-edge (N6,7 and N4,5) EELS spectra
of various single-valent and mixed-valence uranium oxide bulk phases.
Using a low-dose experimental setup, we show that the K-edge theoretical spectra are in excellent agreement with experiment
for both peak positions and relative intensities of respective peaks.
In contrast, U N-edge features are less distinguishing
due to the partially localized nature of the U 5f orbitals and overlapping
multiplet and spin–orbit coupling effects. This work demonstrates
that O K-edge EELS is sufficiently diagnostic to
distinguish a wide range of uranium oxides and that the experimental
approach used here minimizes beam damage and allows valence state
discrimination across the U(IV), U(V), and U(VI) series. When combined
with imaging modes available in electron microscopy, this work enables
a detailed investigation and characterization of uranium redox transformations
at the nanoscale.