Electronic contributions to the high-temperature thermophysical properties of UOsub(2+x): a critical analysis

From an empirical, self-consistent analysis of new high-temperature data (T > 1250 K) on the thermoelectric Seeback coefficient and DC electric conductivity of UO2 the value of the free energy controlling the equilibrium of the thermally induced reaction 2U4+ <--> U3+ + U5+ is determined by treating the electron (U3+) and hole (U5+) excitations of the Mott insulating ground state (U4+) of the material as small, hopping polarons. It is found that the (ambipolar) contribution of this reaction satisfactorily accounts for the upswing in thermal conductivity for T > 2000 K but not for the enthalpy, whose value for T > 2500 K remains anomalously large. Some cautionary remarks are made on the reliability of high-temperature extrapolations, given both the data currently available on relevant thermophysical properties of the material, and the existing degree of theoretical understanding of the processes involved. (author)