Small-polaron hopping in Mott-insulating UO2

Abstract

The DC electrical conductivity in the extrinsic regime of a slightly hyperstoichiometric sample of polycrystalline UO2+x is re-analysed using the general framework of small-polaron theory, taking into account the Mott-insulating nature of the ground state of the stoichiometric material. It is established that above about 150 K the holes move by non-adiabatic hopping, with a mobility activation energy of 0.28+or-0.02 eV. There is no conflict between electrical and thermodynamic data, contrary to some earlier theories. The implications for existing empirical determinations of the Mott-Hubbard gap, U, and the associated entropy of formation of altered-valency cations in the intrinsic regime is investigated, and new (preferred) values presented. The relatively large 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, attention is drawn to some outstanding discrepancies between the semi-empirical values of the small-polaron self-energy and mobility activation energy determined in this work, using Lang-Firsov theory, and those yielded by fully microscopic HADES calculations.