Comparison of the cation valence and coordination in Ce2UO6 and Ce2MoO6

Ce2UO6 and Ce2MoO6 have both been suggested to have significant charge transfer between the metal ions. To quantify this proposed charge transfer, the valence and coordination of cerium and uranium in Ce2UO6 as well as cerium and molybdenum in Ce2MoO6 were studied through a combination of K-, L-, and M-edge XAFS (X-ray absorption fine structure). The results clearly show that these two compounds are not isostructural. Whereas Ce and U in Ce2UO6 are found to disorder on a single site, consistent with the formation of the solid solution Ce0.67U0.33O2, the Ce and Mo coordination is very different. The Mo sits on a site with tetrahedral coordination. The XANES (X-ray absorption near edge structure) data show that for Ce2UO6, Ce and U are both tetravalent, whereas for Ce2MoO6, Ce is trivalent and Mo is hexavalent. There is no evidence in either compound for any significant charge transfer or intermediate valence. The unusual physical properties exhibited by these materials are discussed in light of these findings.