Nonstoichiometry in uranium dioxide

Abstract

A thermodynamic study of the uranium-oxygen system, primarily in the single phase region UO2-UO2,20, has been made at temperatures of 1150° to 1350°K. An electrochemical technique was used to obtain emf measurements on .uranium oxide half-cells. The cells were of the type Fe, FeO|(ZrO2+CaO)|UO2+x, Pt where x varied from 0.01 to 0.20. Partial molar free energies, entropies, and enthalpies of solution of oxygen in UO2+x were calculated. The partial molar free energy is a linear function of temperature at constant composition. The partial molar entropy increases negatively with increasing oxygen content. Equations relating the thermodynamic quantities to the degree of nonstoichiometry have been developed on the assumption that the UO2+x region can be treated as an interstitial solution of oxygen ions in the UO2 fluorite lattice. Semiquantitative agreement with the data was obtained.