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

Abstract

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 (An IV ,An III ) pair and the An IV /An III ratio,Oð2Þ. 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.

Structure of mixed U(IV)–An(III) precursors synthesized by co-conversion methods (where An=Pu, Am or Cm)

“…Under suitable conditions, the oxalate ligand can coordinate a tetravalent actinide and a trivalent one in an original way in the solid. For U(IV)-An(III) mixtures (An = Pu, Am), a new recently described [8,9] mixed hexagonal structure was obtained. The originality of this structure is based on a mixed crystallographic site that can accept either a tetravalent or a trivalent actinide, the charge balance being ensured by adjustment of the single-charged ions within the structure in the second coordination shell.…”

“…The U(IV)-An(III) crystallographic mixed site in this oxalate hexagonal structure is responsible for the homogeneous distribution of actinides in the coprecipitates. Moreover, we established that solid solution domains exist over a wide range of U(IV)/Nd(III) ratios (from 100/0 up to 50/50) for this same structure (Nd used as An(III) surrogate) [8]. This interesting feature can thus also be obtained for the U(IV)/An(III) systems with the trivalent actinide varying from plutonium up to curium (probably with different existence domains due to the decrease in the An(III) ionic radii along the actinide series).…”

Melting behaviour of americium-doped uranium dioxide

“…Average oxidation states of the cations were determined by Linear Combination Fitting (LCF) of the experimental normalised absorption spectra by using well-known reference spectra. The reference compounds used were U +IV O 2.00(1) [34], (U +IV ½ U +V ½ ) 4 O 9 [34] and (U +V ⅔ U +VI ⅓ ) 3 O 8 [35] as well as Am +IV O 2 [36,37] and an oxalate ((U +IV 0.9 Am +III 0.1 ) 2 (C 2 O 4 ) 5 ,6ÁH 2 O) [35,36]. The XANES spectra of the reference compounds have been recorded previously at the same beamline.…”

Structural study of (N2H5,H)2.9U1.1Ce0.9(C2O4)5·10H2O from a conventional X-ray diffraction diagram obtained on a powder synthesized by a fast vortex process

“…Because of the radiotoxicity of actinides [1], process development involves preliminary studies with inactive surrogates containing uranium and a lanthanide. In order to be a representative surrogate, the uranium/lanthanide oxalate must have a structure similar to (NH 4 ) 2 U 2 (C 2 O 4 ) 5 Á 0.7H 2 , oxalate of hexagonal symmetry investigated by Chapelet-Arab [1].…”

“…In order to be a representative surrogate, the uranium/lanthanide oxalate must have a structure similar to (NH 4 ) 2 U 2 (C 2 O 4 ) 5 Á 0.7H 2 , oxalate of hexagonal symmetry investigated by Chapelet-Arab [1].…”