SMALL-POLARON HOPPING IN MOTT-INSULATING UO2

The DC electrical conductivity in the extrinsic regime of a slightly h yperstoichiometric sample of polycrystalline UO2+x is re-analysed usin g the general framework of small-polaron theory, taking into account t he Mott-insulating nature of the ground state of the stoichiometric ma terial. It is established that above about 150 K the holes move by non -adiabatic hopping, with a mobility activation energy of 0.28 +/- 0.02 eV. There is no conflict between electrical and thermodynamic data, c ontrary to some earlier theories. The implications for existing empiri cal determinations of the Mott-Hubbard gap, U, and the associated entr opy of formation of altered-valency cations in the intrinsic regime is investigated, and new (preferred) values presented. The relatively la rge carrier densities predicted near melting necessitate some further consideration of the Coulomb interaction and of its influence both on the magnitude of U and on the mobility activation energy. Finally, att ention is drawn to some outstanding discrepancies between the semi-emp irical values of the small-polaron self-energy and mobility activation energy determined in this work, using Lang-Firsov theory, and those y ielded by fully microscopic HADES calculations.