Electronic structure and stability of hyperstoichiometric UO2+xunder pressure

Abstract

Electronic-Structure and high-pressure phase transitions of stoichiometric uranium dioxide (UO${}_{2}$) and hyperstoichiometric UO${}_{2.03}$ were investigated using first-principles calculations. Density functional theory calculations using the generalized gradient approximation and the projector-augmented wave method with the on-site Coulomb repulsive interactions were applied in order to reasonably calculate the equilibrium volume, total and partial density of states, band gap of UO${}_{2+x}$, and energetics of the high-pressure phase transitions. Structure optimizations were completed with and without the Hubbard $U$-ramping method separately. The $U$-ramping method was intended to remove the metastable states of the 5$f$ electrons of both stoichiometric and hyperstoichiometric UO${}_{2+x}$. Using the Hubbard $U$ parameters ($U$ $=$ 3.8, $J$ $=$ 0.4), whose values are based on the experimental band gap width of 2.1 eV as a reference, the calculated cell parameter for the stoichiometric UO${}_{2}$ with cubic fluorite structure is about 1$%$ greater than the experimental unit cell parameter; in contrast, it is about 1$%$ smaller than the experimental value without the on-site Coulomb repulsive interactions. For hyperstoichiometric UO${}_{2.03}$ with the cubic fluorite structure, the interstitial oxygen at the octahedral interstitial site induces new bands at the top of the band gap of the stoichiometric UO${}_{2}$, similar to those of the high-pressure phase with orthorhombic cotunnite structure. The orbitals associated with the charge transfers to the interstitial oxygen in hyperstoichiometric UO${}_{2.03}$ are partially delocalized and partially localized in both the cubic fluorite structure and orthorhombic cotunnite structure. The energy required for the incorporation of an interstitial O atom is 0.3 eV higher for the orthorhombic phase than for the cubic phase at ambient pressure and increases to 0.5 eV at 10 GPa. The calculated transition pressures from the cubic to the orthorhombic structure are 18 and 27 GPa for UO${}_{2}$ and UO${}_{2.03}$, respectively. The dramatic increase in the calculated transition pressure for the hyperstoichiometric UO${}_{2}$ is related to structural incompatibility of the interstitial oxygen in the cotunnite structure (high-pressure phase), which is less in the case of the fluorite structure. These results suggest that experimentally determined pressure values for the phase transition can be significantly affected by small compositional deviations off the ideal stoichiometric UO${}_{2}$. Comparisons of the results using the $U$-ramping method and without using the $U$-ramping method suggest that the electronic metastability of UO${}_{2}$ affects the calculated total energy and electronic structure and could lead to a different local defect configuration for the hyperstoichiometric orthorhombic phase. However, the metastability has a negligible effect on the calculated phase transition pressure.