Formula omitted] approach on the electronic and thermal properties of hypostoichiometric [formula omitted

Abstract

A Hubbard corrected density functional theory (DFT + U) study of the electronic and thermal properties, of stoichiometric and hypostoichiometric phases, of uranium dioxide (UO2) is presented. Hypostoichiometric phases, with various concentrations of oxygen vacancies in a 2×2×2UO2 supercell, such as UO1.97 (one), UO1.94 (two), UO1.87 (four), UO1.81 (six), and UO1.75(eight vacancies) were considered. The effect of hypostoichiometric variations on thermal conductivity, in particular, the electronic and lattice thermal conductivity has been investigated. At low temperatures (<900 K) phonon scattering from impurities and/or point defects, and at intermediate temperatures (1000–1300 K) from phonons, have a detrimental effects on the lattice thermal conductivity. At considerably high temperatures (>1300 K), the electronic contribution to the thermal conductivity becomes significant. We have analyzed the suppression and enhancement in the lattice thermal conductivity due to presence of oxygen vacancies, in terms of the specific heat capacity, cumulative lattice thermal conductivity and phonon scattering rate. We found a compelling, stimulating, dependency of the electronic and thermal conductivity with the oxygen vacancy concentration. Reduction in the oxygen concentration resulted in a smaller band gap. At low oxygen concentrations, a transition from a wide band gap to a narrow gap and/or metallic nature was determined. The increase in the thermal conductivity was found to be promoted by the substantial increase by the electronic contribution for particular compositions. Our results suggest that, a potential approach for the enhancement of the thermal conductivity at high temperatures, based on hypostoichiometric phases of UO2 in order to obtain characteristics of accident tolerant fuels.