Premelting transition in uranium dioxide

Thermal analysis of the cooling curves of small samples of UO2±x -- initially laser heated (in a high-pressure autoclave to inhibit evaporation) on a sub-second time scale to temperatures just below their melting points [Tm(x)] -- reveals, in the case of nominally stoichiometric UO2.00, a significant, λ-like, heat capacity [Cp(T)] peak near 2,670 K; the cooling curves of samples exposed to a reducing environment, on the other hand, exhibit undercooling, characteristic of a first-order phase transition, while under oxidizing conditions it is found that the premelting transition readily disappears. These findings confirm Bredig's original prediction of a premelting transition in this material, in common with that found in other (nonactinide) fluorites near 0.85Tm. A simple model is presented in terms of which the observed behavior can be rationalized. The model is based on the hypothesis that the premelting transition is due to Frenkel disordering of the oxygen sublattice-a process which is rendered cooperative by attractive interactions between complementary Frenkel defects (oxygen interstitials and vacancies); these interactions are treated in a open-quotes mean-fieldclose quotes approximation. The quantitative degree of maximum disorder (realized just above the transition) is, on the other hand, controlled by repulsive interactions between like defects -- the inclusion of which, solely through their effect on the configurational entropy, satisfactorily reproduces the values inferred from recent high-temperature neutron diffraction experiments. 20 refs., 16 figs., 3 tabs