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 tra nsfer, the valence and coordination of cerium and uranium in Ce2UO6 as well as cerium and molybdenum in Ce2MoO6 were studied through a combi nation of K-, L-, and M-edge XAFS (X-ray absorption fine structure). T he results clearly show that these two compounds are not isostructural . Whereas Ce and U in Ce2UO6 are found to disorder on a single site, c onsistent 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 t etrahedral coordination. The XANES (X-ray absorption near edge structu re) data show that for Ce2UO6, Ce and U are both tetravalent, whereas for Ce2MoO6, Ce is trivalent and Mo is hexavalent. There is no evidenc e in either compound for any significant charge transfer or intermedia te valence. The unusual physical properties exhibited by these materia ls are discussed in light of these findings.