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-Na<sub>2</sub>UO<sub>4</sub>, Na<sub>3</sub>NpO<sub>4</sub>, α-Na<sub>2</sub>NpO<sub>4</sub>, Na<sub>4</sub>NpO<sub>5</sub>, Na<sub>5</sub>NpO<sub>6</sub>, Na<sub>2</sub>PuO<sub>3</sub>, Na<sub>4</sub>PuO<sub>5</sub>, and Na<sub>5</sub>PuO<sub>6</sub> are investigated in this work using X-ray absorption near-edge structure (XANES) spectroscopy at the U-L<sub>3</sub>, Np-L<sub>3</sub>, and Pu-L<sub>3</sub> edges. In addition, a Rietveld refinement of monoclinic Na<sub>2</sub>PuO<sub>3</sub>, in space group <i>C</i>2/<i>c</i>, is reported for the first time, and the existence of the isostructural Na<sub>2</sub>NpO<sub>3</sub> 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.