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

Abstract

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.

New insight in the uranium valence state determination in U Nd1−O2±

“…However, the most used U L 3 -edge XANES can also be strongly affected by the local geometry [16,17]. One major difficulty is often the unavailability of reference compounds with both the correct symmetry and the same valence.…”

A New Look at the Structural and Magnetic Properties of Potassium Neptunate K2NpO4 Combining XRD, XANES Spectroscopy, and Low-Temperature Heat Capacity

“…10 The valence states of the sodium neptunates were confirmed by 237 Np Mössbauer spectroscopy from the values of their isomer shifts, 5−7 while the corresponding XANES spectra were reported in ref (10). The values of the measured inflection points and white lines are reported in Table 3.…”

“…The values of the measured inflection points and white lines are reported in Table 3. The investigated series covered a wide range of oxidation states (IV–VII) and a variety of local coordination geometries around the neptunium cation, i.e., neptunyl (Na 3 NpO 4 and α-Na 2 NpO 4 ), “reverse” neptunyl (Na 4 NpO 5 ), and distorted NpO 6 octahedra (Na 5 NpO 6 ), which has allowed us to correlate the shape of the XANES spectra with the local structural environments, 10 as described later in this paper.…”

“…In addition, the XANES spectrum of K 2 NpO 4 shows the typical double-peaked white lines (WL) of Np(V), Np(VI), and Np(VII) compounds, 10,20,21 while the tetravalent Np IV O 2 compound exhibits a single WL peak. The double peak consists of the main white line at 17620.2(5) eV and a shoulder and reduced peak amplitude about 15 eV above the Np edge.…”

“…The XAS data available on solid actinide compounds with a valence state higher than IV is still very scarce, however, and the relationship between XAS features and electronic density is not fully understood. 10 …”