Synthesis and characterization of mixed An(IV)An(III) oxalates (An(IV) = Th, Np, U or Pu and An(III) = Pu or Am)

The reaction of a solution containing a tetravalent actinide An(IV), a trivalent actinide An(III) and a single-charged cation such as hydrazinium in the presence of oxalic acid in an acidic medium under controlled conditions leads to the precipitation of mixed An(IV)–An(III) oxalate compounds never discussed elsewhere. Two original series were obtained by varying the (AnIV,AnIII) pair and the AnIV/AnIII ratio, (N2H5,H3O)2+xAn2-xIVAnxIII(C2O4)5·4H2O(1) and (N2H5,H3O)1−x[An1-xIIIAnxIV(C2O4)2·H2O]·4H2O(2). The crystal structures were identified from powder diffraction patterns by analogy with hydrazinium uranium (IV) lanthanide (III) oxalates whose structures were solved recently by single crystal X-ray diffraction. Complementary investigations by UV–vis and infra-red spectroscopy and thermogravimetric analysis confirm the presence of both tetravalent and trivalent states of actinides in structures (1) and (2) and the role of single-charged cation and water molecules. The originality of both structures consists in the existence of a mixed crystallographic site for the tetravalent actinide and the trivalent one, the charge balance being ensured by the adjustment of the single-charged ions within the structure. The main difference is that actinides are tenfold coordinated in (1) and ninefold coordinated in (2). This is the first evidence of a mixed actinide(IV)–actinide(III) site in the same oxalate structure.