Thermodynamic Properties of Nonstoichiometric Urania-Thoria Solid Solutions

A solid state electrochemical technique has been used to obtain thermodynamic information on uraniathoria solid solutions of compositions ${\mathrm{U}}_y{\mathrm{Th}}_{\text{1−y}}{\mathrm{O}}_{\text{2+x}}$ with values of y of 0.9 to 0.3 and values of x of 0.02 to 0.16. The electrochemical cells were of the type $\mathrm{Fe},\mathrm{FeO}\mathrm{|\; (}{\mathrm{ZrO}}_2+\mathrm{CaO}\mathrm{)\; |}{\mathrm{U}}_y{\mathrm{Th}}_{\text{1−y}}{\mathrm{O}}_{\text{2+x}}$ , Pt. Measurements were made at temperatures of 1150–1350°K. Partial molar free energies, entropies, and enthalpies of solution of oxygen in the solid solutions were calculated. The partial molar free energy decreases negatively with increasing thorium content for thorium concentrations greater than 30 atomic percent. The partial molar entropy increases negatively with increasing oxygen content or with increasing thorium content. The experimental results are discussed in connection with a mechanism of interstitial solution of oxygen ions in the fluorite lattice. A thermodynamic equation for the entropy is developed which gives semiquantitative agreement with the experimental data.