Premelting transition in uranium dioxide

Thermal analysis of the cooling curves of small samples of UO_2±x—initially laser heated (in a high-pressure autoclave to inhibit evaporation) on a subsecond time scale to temperatures just below their melting points [T_m(x)]—reveals, in the case of nominally stoichiometric UO_2.00, a significant, -like, heat capacity [C_p(T)] peak near 2670 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 0.85T_m. 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 mean-field 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. Assuming that the phase transition in stoichiometric UO_2.00 is of second order, the model predicts a divergent heat capacity, C_v, which approximates well to the experimental (-like) C_p peak. Crucial to reproducing the observed behavior away from stoichiometry is the introduction of a (linear) dependence of the nonconfigurational partial entropy of formation on the prevailing concentration of intrinsic Frenkel defects in UO_2±x; interestingly, it is found that the line of calculated (but unrealized) second-order transitions in UO_2+x intersects the U₄O₉ phase boundary near to where a high-temperature diffuse order-disorder transition has been observed in the oxygen superlattice, suggesting that the second-order, -transition in UO_2.00 is the stoichiometric counterpart of this transition in U₄O₉.