Identification of NpO2+x in the binary Np-O system

Abstract

In contrast to UO 2 and PuO 2 , there is no consensus on the existence of mixed valence NpO 2+x , resulting in a gap between NpO 2 and Np 2 O 5 (the highest binary oxide of Np) in the Np-O phase diagram. We now show NpO 2+x via Np L III Extended X-ray Absorption Fine Structure (EXAFS) spectra of three samples of NpO 2 that, analogous to U and Pu, exhibit multisite Np-O distributions with varying numbers of oxygen atoms at 1.87-1.91 Å. This is supported by the diffraction pattern of the sample with the largest amount of this oxo-type species that can be refined with both the simple fluorite structure and a trigonal one related to -U 4 O 9. The implied Np(V)-bridging oxo moieties as well as possible indications of OH¯ found by detailed EXAFS analysis suggest that NpO 2+x more closely resembles PuO 2+x than UO 2+x. An additional common characteristic suggested by the EXAFS and X-Ray Diffraction (XRD) is the phase separation into NpO 2 and what would be previously unreported Np 4 O 9(-) , indicative of O clustering.

The First Neptunium Dioxide Neutron Diffraction Experiment at HFIR

“…Ten Bragg peaks for NpO 2 were identified, and each 2 θ value listed in Table 1 agreed with X-ray diffraction data. 4 An additional Bragg peak was found at 40.6 2 θ, which corresponds to vanadium metal sample holder.…”

“…However, a recent Extended X-ray Absorption Fine Structure study characterized several samples with varying numbers of oxygen atoms. 4 Additional studies are needed to corroborate these findings. The state-of-the-art neutron capabilities at HFIR can be leveraged to generate new insights into the chemical and magnetic properties of neptunium including the stability of oxidation states, coordination preferences, chemical bonding, and magnetic ground state.…”

Mobility of Aqueous and Colloidal Neptunium Species in Field Lysimeter Experiments

“…The addition of an additional scattering shell at around 1.85 Å (coordination number around 0.5) leaded only to a decrease of the reduced χ 2 from 0.02 to 0.013. Tayal et al, 37 have demonstrated NpO 2+x existence using EXAFS and XRD with an O shell around 1.85−1.9 Å, multisite Np−O. Tayal et al, 37 have demonstrated NpO 2+x existence using EXAFS and XRD with an O shell around 1.85−1.9 Å, multisite Np−O.…”

“…Tayal et al, 37 have demonstrated NpO 2+x existence using EXAFS and XRD with an O shell around 1.85−1.9 Å, multisite Np−O. Tayal et al, 37 have demonstrated NpO 2+x existence using EXAFS and XRD with an O shell around 1.85−1.9 Å, multisite Np−O.…”