Extreme multi-valence states in mixed actinide oxides

In order to assure the safety of oxide-fuel based nuclear reactors, the knowledge of the atomic-scale properties of U1-yMyO2±x materials is essential. These compounds have complex chemical properties, originating from the fact that the earlier actinoids uranium, neptunium, plutonium and americium as well as fission-derived rare earths may occur in different oxidation states. In these mostly ionic materials, aliovalent cationic configurations can induce variation of the oxygen stoichiometry, with dramatic effects on the thermal properties of the fuel. First studies on U1-yAmyO2±x compounds indicated that these materials exhibit particularly complex electronic and local-structure configurations. Here we present an in-depth study of this solid solution, by combining XRD, XAS and Raman spectroscopy to study U1-yAmyO2±x oxides over a wide compositional domain. We present for the first time evidence of the co-existence of four different cations in a fluorite U1-yMyO2±x compound and we illustrate the complex atomic-scale arrangements induced by these extreme multi-valence states.