A solid-state electrochemical device for studying thermodynamic properties of non-stoichiometric oxides: Application to UO2+

We report attempts at determining the evolution of the oxygen activity in hyper-stoichiometric uranium dioxide for very low values of deviation from stoichiometry, using a solid-state electrochemical device. To this end, solid-state oxidation cycles coupled with oxygen activity measurements are carried out on a dense polycrystalline uranium oxide sample. We thus follow the evolution of the oxygen activity in the material as it oxidises or reduces, for temperatures ranging from 1093 to 1273 K. The data are subsequently analysed using an oxygen point defect model and equilibrium constants, corresponding to the various supposed point defect equilibria, are determined. We show that the model faithfully reproduces the trends observed in the experimental data. A more detailed analysis of the equilibrium constants obtained enables us to identify more precisely the limitations of the experimental method. In particular, we highlight the fact that the presence of residual oxygen in the device prevents us from determining the desired physical quantities for stoichiometry deviations less than 10$^-3$ . Ways of circumventing these difficulties which would further allow us to determine point defect diffusion coefficients are outlined.