A defect-thermodynamic approach to PuO 2−x and CeO 2−x

Abstract

The prominent feature of the thermodynamic data of PuO 2− x and CeO 2− x is the strong sigmoidal variation of h ̄ (0 2) and s ̄ (O 2) with x. In this paper, in search for a possible basic mechanism for the onset of such feature of h ̄ (O 2) and s ̄ O 2) in these hypostoichiometric fluorite-type oxides, a defect-thermodynamic formulation is put forward based on the local Pu 3+(Cs 3+)-V o interaction model in the mutual first nearest neighbor coordination sphere. Under some simplified assumptions on the V o arrangement on the oxygen sublattice the number of the local defect—defect configurations is reduced to 16, and using the virtual Gibbs formation energies of these 16 local configurations as the basic parameter the explicit analytical expressions are derived for their concentrations and for the thermodynamic quantities of oxygen ( g ̄ (O 2) , h ̄ (O 2) and s ̄ (O 2) ). The numerical evaluations show that such sigmoidal shape of both h ̄ (O 2) and s ̄ (O 2) as observed in these systems is interpreted and reproduced well by the present approach as a result of the coupled local defect-defect configurational changes between Pu(Ce) and O sites. Discussions are also made on the validity and the limitation of the present model.