Oxygen lattice distortions and U oxidation states in UO 2+x fluorite structures
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1124-Q06-03-O07-03
Oxygen lattice distortions and U oxidation states in UO2+x fluorite structures. Lionel Desgranges1 and Gianguido Baldinozzi2 1 2
CEA, DEN, DEC, Centre de Cadarache, 13108 Saint-Paul-lez-Durance, France Matériaux Fonctionnels pour l‘Energie, SPMS CNRS-Ecole Centrale Paris, 92295 Châtenay-
Malabry, France & CEA/DEN/DANS/DMN/SRMA/LA2M, 91191 Gif-sur-Yvette, France
ABSTRACT The structural changes induced by the changes of the uranium oxidation state in the ideal fluorite lattice of pure UO2 are discussed. Experimental results evidence strong distortions of the oxygen sub-lattice due to dynamic (at high temperature) or static (at low temperature) fluctuations of the local charges in the cationic sublattice. These changes in the oxidation state are often described using the Vegard’s law because a linear dependence of lattice parameter is observed over a wide range of compositions. Nevertheless, an ideal solid solution model cannot explain this behavior where the elastic effects are directly related to the ionic radii of the cations. Strong evidence is provided that enthalpy effects are relevant in these systems and that they are directly associated with the local structural changes observed during neutron scattering experiments.
INTRODUCTION During its irradiation in power reactors, uranium dioxide, the most used nuclear fuel, undergoes not only displacive irradiation damage but also chemical changes because of the large range of oxidation states of U atoms interacting with the elements generated by the nuclear fissions. Amongst these fission products, lanthanides (Ln) are reported in literature [1-3] to form (U,Ln)O2+/-x solid solutions with x near to 0. It is generally admitted that the evolution of the lattice parameter of U based fluorite type structures follows a linear behaviour over a large range of dopant concentrations. For instance, the unit cell parameter of (U,Ln)O2 binary systems, for Ln= Gd [1],La [2], Nd usually verifies the Vegard’s law up to 50% [3]. From this experimental result, it was generally assumed that the incorporation of aliovalent cations (U with a different oxidation state or a Ln element) proceeds by a one to one substitution on the uranium site. Thus, the ionic radius of the aliovalent cation would be the most significant parameter to predict its actual substitution for a U atom in the cation sublattice of UO2 [4,5]. However, from a chemical point of view, the assumption of uncorrelated vacancies in the oxygen sub-lattice over such a large range of compositions is questionable and the model of ideal solid solution is certainly not fulfilled for these binary systems. The coordination polyhedron of the lanthanide and the one of uranium, which may change its oxidation state to compensate lanthanide incorporation, are very likely to be distorted. In UO2, increasing temperature or changing the oxygen concentration modifies the oxidation state of uranium. In this paper, we demonstrate first that in the very simple case of
stoichiometric UO2 having uranium atoms with diffe
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