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

Tayal, Akhil; Conradson, Steven D.; Baldinozzi, Gianguido; Namdeo, Sonu; Roberts, Kevin E.; Allen, Patrick G.; Shuh, David K.

doi:10.1016/j.jnucmat.2017.04.019

Title: Identification of NpO_2+x in the binary Np-O system

Journal Article · 14 April 2017 · Journal of Nuclear Materials

DOI: https://doi.org/10.1016/j.jnucmat.2017.04.019 · OSTI ID:1510745

Tayal, Akhil ^[1]; Conradson, Steven D. ^[2];

^[3]; Namdeo, Sonu ^[4]; Roberts, Kevin E. ^[5]; Allen, Patrick G. ^[5]; Shuh, David K. ^[6]

Synchrotron SOLEIL, Gif-sur-Yvette Cedex (France)
Josef Stefan Institute, Ljubljana (Slovenia); Washington State Univ., Pullman, WA (United States)
Chatenay-Malabry and CEA/DEN/DANS/DMN/SRMA/LA2M-LRC CARMEN, Gif-sur-Yvette (France)
Devi Ahilya Vishwavidyalaya, Indore (India)
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

In contrast to UO₂ and PuO₂, there is no consensus on the existence of mixed valence NpO_2+x, resulting in a gap between NpO₂ and Np₂O₅ (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 NpO₂ that, analogous to U and Pu, exhibit multisite Np-O distributions with varying numbers of oxygen atoms at 1.87–1.91 Å. Furthermore 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 α-U₄O₉. 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 NpO₂ and what would be previously unreported Np₄O_9(-δ), indicative of O clustering.