Structural study of (U 0.90Ce 0.10) 4O 9− δ, an anion-excess fluorite superstructure of U 4O 9− δ type

Abstract

In the U–Ce–O system, a solid solution (U,Ce)O 2+ x of fluorite type containing anionic excess is known in a wide composition range. For high values of x, it transforms to a (U 1− y Ce y ) 4O 9− δ phase deriving from the β-U 4O 9− δ type [ordered anion-excess fluorite superstructure phase; I-43 d space group; a=21.7484(1) Å for y=0.10]. The crystal structure of (U 0.9Ce 0.1) 4O 9− 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–U 4O 9− δ and not fully confirmed till now, has been verified. The structure is based on an ordered distribution of cuboctahedral clusters U 6O 37 inside a fluorite matrix. A preferential ordering of Ce 4+ (and U 4+) 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 M 64O 143 ( MO 2.234) and no traces of excess anions, completing the supposed composition up to M 4O 9, could be detected.