Structural study of (U0.90Ce0.10)4O9−δ, an anion-excess fluorite superstructure of U4O9−δ type

In the U–Ce–O system, a solid solution (U,Ce)O2+x of fluorite type containing anionic excess is known in a wide composition range. For high values of <f>x</f>, it transforms to a (U1−yCey)4O9−δ phase deriving from the β-U4O9−δ type [ordered anion-excess fluorite superstructure phase; <f>I</f>-<f>43d</f> space group; <f>a=21.7484(1)</f> A˚ for <f>y=0.10</f>]. The crystal structure of (U0.9Ce0.1)4O9−δ has been refined by the Rietveld method on a powder sample measured on D2B at ILL Grenoble. The structural model, proposed by Bevan et al. for β–U4O9−δ and not fully confirmed till now, has been verified. The structure is based on an ordered distribution of cuboctahedral clusters U6O37 inside a fluorite matrix. A preferential ordering of Ce4+ (and U4+) on the so-called “centaur polyhedra” with 10 coordination is proposed, on the basis of bond valence calculations. The structure so determined has the composition M64O143 (MO2.234) and no traces of excess anions, completing the supposed composition up to M4O9, could be detected. [Copyright &y& Elsevier]