Heat capacity measurement and EXAFS study of (U0.85Mg0.15)O2− x for x =0 and 0.11

Heat capacities and electrical conductivities of (U.ssMg. js)O, , (.v=0 and 0.11 were measured simultaneously by means of a direct heating pulse calorimeter (DH PC) in the temperature range from 301} to 1500 K. Anomalot.s increases in the heat capacity curves of ( U.ss Mgot5 ) Oe ., (x = 0 and 0.1 I were observed above about 800 and 1150 K, respectively. The values [br the enthalpy of oxygen Frenkel defect [ormation were calculated from the excess heat capacity and were Ibund to be similar to those tbr UO, doped with rare earth elements. On the other hand. no anomaly was seen in the electrical conductivity curve around tile onset [enlperature of the arll,)lllalous increase ill the heal capacity. It was, therelbrc, concluded that the excess heat capacity originates fronl the predominant contribution of the Ibrmation of Frenkel pair-like defects of oxygen. All extended X-ray absorption fine structure (EXAFS) experiment shows a difli:rent environment of oxygen around uranium and magnesium, and this should be a cause of the onset temperature difference.