Chemical thermodynamic representations of and

All available oxygen potential-temperature-composition data for the calcium fluorite-structure sup(**) phase were retrieved from the literature and utilized in the development of a binary solid solution representation of the phase. The data and phase relations are found to be best described by a solution of [Pusub(4/3)O2] and [PuO2] with a temperature dependent interaction energy. The fluorite-structure is assumed to be represented by a combination of the binaries and , and thus treated as a solution of [Pusub(4/3)O2], [PuO2], [UO2], and either [U2Osub(4.5)] or [U3O7]. The resulting equations well reproduce the large amount of oxygen potential-temperature-composition data for the mixed oxide system, all of which were also retrieved from the literature. These models are the first that appear to display the appropriate oxygen potential-temperature-composition and phase relation behavior over the entire range of existence for the phases. (orig.)