Analysis of recent measurements of the heat capacity of uranium dioxide

Abstract

Recent measurements of the heat capacity of UO 2 are analysed in the temperature range from room temperature to 8000 K. The high temperature behaviour of c p( T) is essentially governed by heat energy exchange mechanisms in the solid, involving the formation of atomic and electronic defects. The analysis is centred on the λ transition observed at T ≅ 0.8 T m, which is interpreted in terms of the cooperative formation of anion Frenkel defects. Electronic defects, in the form of localized small polarons, formed in the same temperature range, are shown to provide only a minor contribution to c p, owing to electron-hole interactions. Schottky defects are predicted to play an effective role only between 2700 K and T m. The decrease in the heat capacity above the melting point of the material is attributed principally to the saturation of the thermally activated contributions. The heat capacity in the liquid is sustained by molecular vibrations up to 4500 K; only at higher temperatures do more energetic electronic excitations take place. The formation of defects in the solid is described by using meanfield models which are able to reproduce the observed transition and its dependence on the stoichiometry in UO 2 ± x .