High temperature heat capacities of (U0.91 M0.09)O2 (where M is Pr, Ce, Zr) from 290 to 1410 K

Abstract Heat capacities of (U 0.91 M 0.09 )O 2 , where M is Pr, Ce and Zr, were measured by direct heating pulse calorimetry over the temperature range 290–1410 K. No anomalous increase in the heat capacity curve of each sample was observed at temperatures up to 1410 K, dissimilarly to the cases of (U 1− y M y )O 2 (M is a trivalent cation (Gd, Eu, La) or simulated fission product composed of several tri- and tetravalent cations; y = 0.044–0.142) found previously by the present authors. It is proposed that the occurrence of the heat capacity anomaly of (U,M)O 2 , where M is a trivalent cation or simulated fission product, originates from the predominant contribution of Frenkel pair-like defects of oxygen formed by the introduction of aliovalent cations (M 3+ ) in UO 2 from the electroneutrality condition, and that the introduction of the tetravalent cations which are the same valency as uranium ions in UO 2 results in insignificant effect on the total number of the oxygen defect, producing no heat capacity anomaly. The difference in the onset temperatures of the heat capacity anomaly of UO 2 doped with various trivalent cations is discussed from the viewpoint of the difference between the averaged cationoxygen interatomic distance calculated from the ionic radii and that obtained from the experimental lattice constants on the assumption of the perfect fluorite structure, reflecting the variety of the distribution of oxygen around cations in the lattice, i.e. that of the local structural environments for cationoxygen bonding. A linear relation between the onset temperature of the heat capacity anomaly and the difference in the cationoxygen interatomic distance was found, supporting the importance of the variety of local structures in UO 2 as the origin of the heat capacity anomaly.