  Structural characterization of actinide nanoparticles (NPs) is of primary
importance and hard to achieve, especially for non-homogeneous samples with NPs
below 3 nm. By combining High Energy X-ray Scattering (HEXS) and
High-Energy-Resolution Fluorescence Detected X-ray Near-Edge Structure (HERFD
XANES), we characterized for the first time both short- and medium-range order
of ThO2 NPs obtained by chemical precipitation. With this methodology, a novel
insight into the structure of NPs at different steps of their formation process
is achieved. The Pair Distribution Function (PDF) reveals a high concentration
of ThO2 small units similar to Th hexamer clusters mixed with 1 nm ThO2 NPs in
the initial steps of formation. Drying the precipitates at 150 C promotes
recrystallization of the smallest units into more thermodynamically stable ThO2
NPs. HERFD XANES at Th M4 edge, a direct probe of the f states, shows
variations that we correlate to the break of the local symmetry around Th
atoms, which most likely concerns surface atoms. Together, HEXS and HERFD are a
powerful methodology to investigate actinide NPs and their formation mechanism.
