Extreme multi-valence states in mixed actinide oxides

Abstract

To assure the safety of oxide-fuel based nuclear reactors, the knowledge of the atomic-scale properties of U 1−y M y O 2±x materials is essential. These compounds show complex chemical properties, originating from the fact that actinides and rare earths may occur with different oxidation states. In these mostly ionic materials, aliovalent cationic configurations can induce changes in the oxygen stoichiometry, with dramatic effects on the properties of the fuel. First studies on U 1−y Am y O 2±x indicated that these materials exhibit particularly complex electronic and local-structure configurations. Here we present an in-depth study of these compounds, over a wide compositional domain, by combining XRD, XAS and Raman spectroscopy. We provide evidences of the coexistence of four different cations (U 4+ , U 5+ , Am 3+ , Am 4+) in U 1−y M y O 2±x compounds, which nevertheless maintain the fluorite structure. Indeed, we show that the cationic sublattice is basically unaffected by the extreme multi-valence states, whereas complex defects are present in the oxygen sublattice.

High-energy resolution X-ray spectroscopy at actinide M4,5 and ligand K edges: what we know, what we want to know, and what we can know

“…The DFT+U approach was used with the help of Quantum Espresso v. In case of Am-doped UO 2 , the situation was found to be different. It was reported that americium introduced into the UO 2 lattice is predominantly in the Am(III) state [111][112][113][114][115][116][117][118]…”

“…using the XAS technique at the U L 3 edge 112 . Although, the possibility of the existence of complex defects in oxygen sublattice upon Am doping was discussed 117 the involvement of oxygen in the charge compensation mechanism in terms of changes in the electronic structure was not analyzed.…”

ROBL-II at ESRF: a synchrotron toolbox for actinide research

“…X-ray absorption fine-structure (XAFS) spectroscopy, including (conventional) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectroscopies, has been since the early 1990s one of the most universal methods to study actinide speciation in liquids, solids and at interfaces, with applications including fundamental, coordination, environmental and materials chemistry of actinides (Nitsche, 1995;Denecke, 2006;Geckeis et al, 2013;Maher et al, 2013;Epifano et al, 2019). In fact, the first version of ROBL built in 1997 was dedicated to XAFS as a sole method to study actinide chemistry (Reich et al, 2000).…”

X-ray absorption spectra of f-element complexes: insight from relativistic multiconfigurational wavefunction theory

“…[46][47][48][49][50][51] XANES at the metal L and M edges is also used in Ln and An chemistry research. 26,36,[52][53][54][55][56][57][58][59][60][61][62][63][64][65] The L edge probes dipole-allowed 2p → nd transitions and also provides information about the fshell occupation. The (relative) intensity between the spectral features, and the energy of the main edge, are very sensitive to the metal oxidation state and f-electron count.…”