Figure 2
Model fit to the experimental data on \({\Delta G}_{{\mathrm{O}}_{2}}=\mathit{RT}\mathrm{ln}({P}_{{\mathrm{O}}_{2}}/{P}^{0})\) vs. \(\delta =(x-y)/2\) for the system GdO1.5-UO2-UO3. The experimental data are from Lindemer & Sutton36. Note, that δ can be written as δ = (x − y)/2, as it is a function of the mole fractions, x and y, of the UO2.5 and LnO1.5 endmembers. δ can be also evaluated as δ = O/M − 2. A negative/positive deviation from δ = 0 implies the presence of either oxygen vacancies or oxygen interstitials. Note also, that δ characterizes non-stoichiometry only of a mono-phase fluorite, while O/M − 2 is also applicable to a poly-phase system. \({\Delta G}_{{\mathrm{O}}_{2}}\) is a convenient function to visualize effects of the temperature and/or the partial pressure of O2 on the chemical potential of O2, while the dimensionless quantity \(\mathrm{log}({P}_{{\mathrm{O}}_{2}}/{P}^{0})\) is more convenient when the temperature and the pressure effects need to be distinguished. Both quantities are used throughout the text.
