Insights into the Thermal Decomposition of Lanthanide(III) and Actinide(III) Oxalates – from Neodymium and Cerium to Plutonium

Abstract

Lanthanides are often used as surrogates to study the properties of actinide compounds. Their behaviour is considered to be quite similar as they both possess f valence electrons and are close in size and chemical properties. This study examines the potential of using two lanthanides (neodymium and cerium) as surrogates for plutonium during the thermal decomposition of isomorphic oxalate compounds, in the trivalent oxidation state, into oxides. Thus, the thermal decomposition of neodymium, cerium and plutonium(III) oxalates are investigated by several coupled thermal analyses (TG-DTA/DSC-MS/FTIR) and complementary characterisation [a] CEA Marcoule, Nuclear Energy Division, DRCP/SCPS/LC2A, 4986techniques [XRD, UV/Vis and FTIR spectroscopy, scanning electron microscopy (SEM), carbon analysis] under both oxidizing and inert atmospheres. The thermal decomposition mechanisms determined in this study confirmed some previously reported results, among diverging propositions, and also elucidated some original mechanisms not previously considered. The calculated thermodynamic and kinetic parameters for the studied systems under both atmospheres are reported and compared with available literature data. Similarities and differences between the thermal behaviour of plutonium(III) and lanthanide(III) oxalates are outlined.The residual carbon contents in the neodymium and cerium oxides obtained after calcination of the oxalate pre-Eur.

From uranium(IV) oxalate to sintered UO 2 : Consequences of the powders' thermal history on the microstructure

“…Nevertheless, the heat treatments combining the use of oxidizing and reducing atmospheres appeared to be much more efficient to operate the carbon elimination. Again, this observation could be correlated with the Boudouard equilibrium that led to the disproportionation of CO into C and CO 2 under inert atmosphere [36]. In these conditions, the amount of carbon still remaining in the samples heated in reducing atmosphere was systematically found to be 4 to 5 times higher than that measured in samples previously calcined in air.…”

“…Nevertheless, if the dehydration step still occurred in the same range of temperature, the decomposition of oxalate species was found to take place at lower temperature as it ended below 350 • C. Such difference could be ascribed to the more efficient elimination of the carbon species in air. Indeed, in such conditions, CO coming from the decomposition of C 2 O 4 species is mainly oxidized by O 2 while it tends to disproportionate under inert or reducing atmosphere to form CO 2 and elemental carbon (Boudouard equilibrium) [36]. Moreover, an additional weight loss was pointed out at around 650 • C and could be ascribed to the transformation of an intermediate uranium carbonate or oxocarbonate into the final oxide product, probably as U 3 O 8 .…”

A low-temperature synthesis method for AnO2 nanocrystals (An = Th, U, Np, and Pu) and associate solid solutions

“…For the Pu-containing compositions, the formation of the Pu III -oxalate intermediate might be possible. 20,21 Finally, the resulting NCs were washed with water, ethanol, and acetone, in order to gradually decrease the polarity of the solution. In the case of the uranium-containing samples, the work was performed under argon and using minute amounts of hydrazine, in order to limit the oxidation of U IV .…”

Preparation of rare earth oxides from Synchysite oxidized ore by acid leaching

“…The low temperature calcined oxide powders generally kept their original morphology of the precipitated oxalates [11]. Cerium oxalate with the layered structure, which mostly kept its morphology by calcination up to 1000°C, was also showed previously [12]. Therefore, it can be suggested that the mixed rare earth oxalate could show layered structure, though; we haven't observed it by an analytical technique.…”