Study of Nonstoichiometry of 〈U1–zGdzO2±x〉

The extensive nonstoichiometry in the 〈U1–zGdzO2±x〉† phase was investigated experimentally and the data are represented by a chemical thermodynamic method. The experimental ranges of temperature, oxygen potential, and z were 1273 to 1773 K, 0 to −600 kJ/mol, and 0.1 to 0.8, respectively. For hypostoichiometry, ideal-solution thermodynamics for the equilibrium 3Gd4/3O2+ 4UO2+ (O2) = 6U2/3Gd2/3O8/3 were used to represent the experimental data, while for hyperstoichiometry a nonideal solution was used for the equilibrium 4UO2+ (O2) = 2U2O5. The wide ranges in x and z led to an improvement of the previous analysis of literature data and led to partial molal Gibbs free energy values that are useful for any thermodynamic calculation involving the phase.

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