Nonstoichiometry in uranium dioxide

Abstract

Abstract Thermodynamic properties of uranium oxides with O/U atomic ratios between 2.04 and 2.34, principally in the nonstoichiometric UO 2+x single-phase region, were determined at temperatures in the range 500°C to 1100°C by electromotive force measurements with cells of the type Ni-NiO¦ZrO 2 (+CaO)¦uranium oxide. The relative partial molar free energies of oxygen for the single-phase region UO 2+x and the two two-phase coexisting regions UO 2+x -U 4 O 9−y and U 4 O 9 -U 3 O 8−z were obtained with adequate precision. Several statistical models for the defect structure of UO 2+x have been reviewed. The variation of the relative partial molar entropy of oxygen with composition in the UO 2+x phase was derived on the basis of the 2:2:2 defect complex model proposed from the neutron diffraction study of Willis. The entropies were computed to fit the experimental data assuming that the excess oxygen atoms entered into the fluorite structure in pairs and that the available sites for such oxygen pairs and U 5+ ions would be partially restricted for occupancy.