Oxalates are frequently used as precipitating agents for tri- and tetravalent actinides in the nuclear fuel cycle. For example, plutonium has been purified by the precipitation of plutonium(IV) oxalate for over 60 years, and a common route for the production of ThO2 involves the calcination of thorium(IV) oxalate. Despite this, the crystal structures of Pu(C2O4)2.6H2O and Th(C2O4)2.6H2O and other relevant compounds to the above processes, have yet to be reported. The structures of their isomorphs, Np(C2O4)2.6H2O and U(C2O4)2.6H2O, were reported in 1997 and 2008, respectively, and subsequent work relating to the Pu and Th structures assumed isomorphism with the previously reported U and Np structures. In this work, four actinide(IV) oxalates, An(C2O4)2.6H2O (An = Th(IV), U(IV), Np(IV), Pu(IV)), were synthesized and characterized. The structures of the Th and Pu oxalates are reported for the first time. Additionally, the structures of the U and Np oxalates were re-evaluated along with the Th and Pu compounds, and the data collected within this work revealed insights regarding the coordination of waters to the metal centers as well as a change in orientation of the coordinated oxalates.

Insights into the structural ambiguity of An(C2O4)2.6H2O sheets (An = Th(IV), U(IV), Np(IV), Pu(IV))

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