Structural Investigation of Uranium–Neptunium Mixed Oxides Using XRD, XANES, and ¹⁷O MAS NMR

Abstract

Uranium−neptunium mixed dioxides are considered as fuels and targets for the transmutation of the minor actinides in fast neutron reactors. Hereafter, a local and atomic scale structural analysis was performed on a series of U 1−x Np x O 2 (x = 0.01; 0.05; 0.20; 0.50; 0.75; 0.85) synthesized by the sol−gel external gelation method, for which longer range structural analysis indicates that the process yields solid solutions. The oxidation state of IV for uranium and neptunium cations was confirmed using U L III and Np L III edge X-ray absorption near edge structure (XANES). The atomic scale structure was probed with 17 O magic angle spinning nuclear magnetic resonance (MAS NMR) for the anion. Structural distortions due to the substitution of U by the smaller Np cation were detected by 17 O MAS NMR.

Fingerprint of local disorder in long range ordered isometric pyrochlores

“…For La 2 Zr 2 O 7 , this attribution is confirmed by the relative intensity between peaks B and A which is 6:1 as expected from its crystal structure. Even if the 17 O MAS NMR spectrum of Nd 2 Zr 2 O 7 is only qualitative (as it was acquired using two offsets), the large 17 O NMR shift of peak C conforms with O atoms surrounded by four Nd 3+ spin bearing centres with unpaired electrons (see Supplementary Note 1) 47–50 . The Raman spectra of La 2 Zr 2 O 7 and Nd 2 Zr 2 O 7 can both be reproduced by a set of five Lorentzian bands corresponding to the most obvious vibrational modes 28 .…”

Investigating lanthanide dopant distributions in Yttrium Aluminum Garnet (YAG) using solid state paramagnetic NMR

“…The interactions of an NMR-observed nuclide with the unpaired electron spins from a neighboring ion can result in spectra that can give quantitative insight into local environments. Often termed 'paramagnetic NMR' , this approach has been applied to a variety of systems, including battery materials, fuel cell materials, natural and synthetic minerals, and simple oxides doped with transition metals, lanthanides, and even actinides [6][7][8][9]. There are two primary mechanisms that describe such interactions: the pseudocontact shift, in which the effective dipolar magnetic field produced by the unpaired electrons (necessarily in low-symmetry environments) affects the observed nucleus directly through space, and the Fermi contact shift, where unpaired electron density interacts directly or indirectly with s shells of the observed nucleus via interatomic bonds [10].…”

Structural Investigation of (U0.7Pu0.3)O2-x Mixed Oxides

“…This phenomenon has been already observed in other materials when comparing pure compounds and solid solutions. 23,24 Characterization of Substoichiometric MOX. The substoichiometric MOX U 0.7 Pu 0.3 O 2-x sample presents a biphasic composition (Figure 4), in agreement with previously reported experimental data 2,5,26,27 and the thermodynamic model.…”