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Identification of NpO2+xin the binary Np-O system

  • Author(s): Tayal, A
  • Conradson, SD
  • Baldinozzi, G
  • Namdeo, S
  • Roberts, KE
  • Allen, PG
  • Shuh, DK
  • et al.
Abstract

© 2017 Elsevier B.V. In contrast to UO2and PuO2, there is no consensus on the existence of mixed valence NpO2+x, resulting in a gap between NpO2and Np2O5(the highest binary oxide of Np) in the Np-O phase diagram. We now show NpO2+xvia Np LIIIExtended X-ray Absorption Fine Structure (EXAFS) spectra of three samples of NpO2that, 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 α-U4O9. The implied Np(V)-bridging oxo moieties as well as possible indications of OH¯ found by detailed EXAFS analysis suggest that NpO2+xmore closely resembles PuO2+xthan UO2+x. An additional common characteristic suggested by the EXAFS and X-Ray Diffraction (XRD) is the phase separation into NpO2and what would be previously unreported Np4O9(–δ), indicative of O clustering.

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