Structural Properties and Charge Distribution of the Sodium Uranium, Neptunium, and Plutonium Ternary Oxides: A Combined X‑ray Diffraction and XANES Study

The charge distributions
in α-Na2UO4, Na3NpO4, α-Na2NpO4, Na4NpO5, Na5NpO6, Na2PuO3, Na4PuO5, and Na5PuO6 are investigated
in this work using X-ray absorption near-edge structure (XANES) spectroscopy
at the U-L3, Np-L3, and Pu-L3 edges.
In addition, a Rietveld refinement of monoclinic Na2PuO3, in space group C2/c, is
reported for the first time, and the existence of the isostructural
Na2NpO3 phase is revealed. In contrast to measurements
in solution, the number of published XANES data for neptunium and
plutonium solid phases with a valence state higher than IV is very
limited. The present results cover a wide range of oxidation states,
namely, IV to VII, and can serve as reference for future investigations.
The sodium actinide series show a variety of local coordination geometries,
and correlations between the shape of the XANES spectra and the local
structural environments are discussed herein.