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

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 techniques [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 thermal decomposition mechanisms of neodymium, cerium and plutonium(III) oxalates under inert and oxidising atmospheres are determined and compared, thermodynamic and kinetic considerations are proposed and the different phenomena controlling the residual carbon content in the oxides are outlined.