A structural-thermodynamic model of the (U1-zPuz)O2+δ solid solution

Thermodynamic mixing properties of the (U 1− z Pu z )O 2+ δ fluorite solid solution are modelled by taking into account the dependence of the δ parameter on the chemical potential of O 2 . Simultaneously, the model is made consistent with phase separation data that are relevant for the hypo-stoichiometric domain. An ion-packing approach is used to describe the variation in the lattice parameter as functions of z and δ . The linking of the ion-packing and thermodynamic models allows predicting the lattice parameter variation across the miscibility gap. The constructed diagrams provide means for estimating equilibration temperatures of biphasic assemblages from X-ray diffraction data.