Toshiharu Ohnuma

A first-principles calculation for uranium dioxide (UO2) in an antiferromagnetic structure with four types of point defects, uranium vacancy, oxygen vacancy, uranium interstitial, and oxygen interstitial, has been performed by the projector-augmented-wave method with generalized gradient approximation combined with the Hubbard U correction. Defect formation energies are estimated under lattice relaxation for supercells containing 1, 2, and 8 unit cells of UO2. The electronic structure, the atomic displacement and the stability of defected systems are obtained, and the effects of cell sizes on these properties are discussed. The results form a self-consistent dataset of formation energies and atomic distance variations of various point defects in UO2 with relatively high precision. We show that a supercell with 8 UO2 unit cells or larger is necessary to investigate the defect behavior with reliable precision, since point defects have a wide-ranging effect, not only on the first nearest neighbor atoms of the defect, but on the second neighbors and on more distant atoms.

A comprehensive investigation on point defects and their clustering behavior in nonstoichiometric uranium dioxide $\mathrm{U}{\mathrm{O}}_{2\ifmmode\pm\else\textpm\fi{}x}$ is carried out using the $\mathrm{LSDA}+\mathrm{U}$ method based on density functional theory. Accurate energetic information and charge transfers available so far are obtained. With these energies that have improved more than 50% over that of pure generalized gradient approximation and local density approximation, we show that the density functional theory predicts the predominance of oxygen defects over uranium ones at any compositions, which is possible only after properly treating the localized $5f$ electrons. Calculations also suggest an upper bound of $x\ensuremath{\sim}0.03$ for oxygen clusters to start off. The volume change induced by point uranium defects is monotonic but nonlinear, whereas for oxygen defects, increasing $x$ always reduces the system volume linearly, except dimers that require extra space for accommodation, which has been identified as a metastable ionic molecule. Though oxygen dimers usually occupy Willis ${\mathrm{O}}^{\ensuremath{''}}$ sites and mimic a single oxygen in energetics and charge state, they are rare at ambient conditions. Its decomposition process and vibrational properties have been studied carefully. To a general clustering mechanism in anion-excess fluorites systematically obtain, we also analyze the local stabilities of possible basic clustering modes of oxygen defects. The result shows a unified way to understand the structure of Willis-type and cuboctahedral clusters in $\mathrm{U}{\mathrm{O}}_{2+x}$ and $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{U}}_{4}{\mathrm{O}}_{9}$. Finally, we generalize the point defect model to the independent cluster approximation to include clustering effects; the impact on defect populations is discussed.

We investigated the intrinsic defect formation energy and oxide-ion migration mechanism in Gd2Ti2O7 pyrochlore. It was found that the vacancy formation energy of Gd is lower than that of Ti. For the oxygen vacancy, O(48f) was found to show lower vacancy formation energy than O(8b). The formation energy of the vacancy complex showed that the Gd vacancy is accompanied with the O(48f) vacancy, which is consistent with our experiment. The migration energy of O(48f) along the <100> direction, which is dominant migration path for ionic conduction, was calculated to be 0.43 eV. On the other hand, we found that Gd vacancy increases O(48f) migration energy. For example, the migration energy of O(48f) along the <100> direction was increased to be 1.36 eV by the local compressive strain around Gd vacancy. This finding could explain our previous experimental result of decreasing conductivity with increasing Gd deficiency. Along with the oxide-ion migration mechanism in Gd2Ti2O7, O(48f) migration energies along both <100> and <110> directions for various A2B2O7 pyrochlore structures were investigated. As a general trend of oxide-ion migration in the pyrochlore structure, we propose that O(48f) migration along the <100> direction is governed by the strength of B–O bonding. On the other hand, the ratio of ionic radius B/A is proposed to determine O(48f) migration along the <110> direction in A2B2O7 pyrochlore.