A DEFECT-THERMODYNAMIC APPROACH TO PUO2-X AND CEO2-X

The prominent feature of the thermodynamic data of PuO2-x and CeO2-x i s the strong sigmoidal variation of h(O2)BAR and s(O2)BAR with x. In t his paper, in search for a possible basic mechanism for the onset of s uch feature of h(O2)BAR and s(O2)BAR in these hypostoichiometric fluor ite-type oxides, a defect-thermodynamic formulation is put forward bas ed on the local Pu3+(Ce3+)-V(o) interaction model in the mutual first nearest neighbor coordination sphere. Under some simplified assumption s on the V(o) arrangement on the oxygen sublattice the number of the l ocal defect-defect configurations is reduced to 16, and using the virt ual Gibbs formation energies of these 16 local configurations as the b asic parameter the explicit analytical expressions are derived for the ir concentrations and for the thermodynamic quantities of oxygen (g(O2 )BAR, h(O2)BAR and s(O2)BAR). The numerical evaluations show that such sigmoidal shape of both g(O2)BAR and s(O2)BAR as observed in these sy stems is interpreted and reproduced well by the present approach as a result of the coupled local defect-defect configurational changes betw een Pu(Ce) and O sites. Discussions are also made on the validity and the limitation of the present model.