Multiscale structural characterizations of mixed U(IV)-An(III) oxalates (An(III) = Pu or Am) combining XAS and XRD measurements

Mixed actinides(III,IV) oxalates of general formula M2.2UIV1.8AnIII0.2(C2O4)5·nH2O (An = Pu or Am and M = H3O+ and N2H5+) have been quantitatively precipitated by oxalic precipitation in nitric medium (yield> 99%). Thorough multiscale structural characterizations by XRD and XAS measurements evidenced the existence of oxalate solid solutions. The XANES analysis confirmed that the oxidation states of the metallic cations, tetravalent for uranium and trivalent for plutonium and americium are kept during the precipitation step. The local environments around U+IV, Pu+III and Am+III determined by EXAFS measurements are similar and show that cations are surrounded by five oxalate ligands. The mean metal-oxygen distances by XAS measurements are in agreement with those calculated from XRD lattice parameters. Finally, these An(III)-An(IV) oxalate solid solutions can be used to synthetize mixed actinide oxides characterized by a high homogeneity at molecular scale.