Mechanistic insight into oxygen vacancy migration in SrFeO3-δ from DFT+U simulations.

SrFeO3-δ is known to be an effective oxygen ion conductor and oxygen vacancies are central to its performance. SrFeO3-δ displays four crystallographic structural transitions as it undergoes oxygen reduction over a broad range of operating temperatures. In this work, systematic density functional theory calculations using the Hubbard U correction were performed to understand oxygen vacancy interactions and migration as a function of vacancy concentrations in SrFeO3-δ (δ = 0-0.5). We found strong repulsion between oxygen vacancies at close distance while these oxygen vacancies are stabilized at further distance. We also found that the oxygen migration is highly anisotropic and the calculated effective migration energy for the oxygen migration tends to be high and increases from 0.91 eV to 1.30 eV as δ goes from 0.125 (tetragonal phase) to 0.25 (orthorhombic phase). In the ordered brownmillerite SrFeO2.625, the oxygen migration is restricted in the one-dimensional channel because of the highly anisotropic nature of the crystal structure, resulting in the relatively low effective migration energy of 0.49 eV. This explains the experimental activation energy of 0.55 ± 0.05 eV. These results suggest the importance of regulating the oxygen migration path via the crystal structure design toward development of a SrFeO3-δ based fast oxygen conductor.

First-principles calculation on oxygen ion migration in alkaline-earth doped La2GeO5.

By using first-principles calculations based on the density functional theory, we investigated the doping effects of alkaline-earth metals (Ba, Sr and Ca) in monoclinic lanthanum germanate La2GeO5 on its oxygen ion conduction. Although the lattice parameters of the doped systems changed due to the ionic radii mismatch, the crystal structures remained monoclinic. The contribution of each atomic orbital to electronic densities of states was evaluated from the partial densities of states and partial charge densities. It was confirmed that the materials behaved as ionic crystals comprising of cations of La and dopants and anions of oxygen and covalently formed GeO4. The doping effect on the activation barrier for oxygen hopping to the most stable oxygen vacancy site was investigated by the climbing-image nudged elastic band method. By tracing the charge density change during the hopping, it was confirmed that the oxygen motion is governed by covalent interactions. The obtained activation barriers showed excellent quantitative agreements with an experiment for the Ca- and Sr-doped systems in low temperatures as well as the qualitative trend, including the Ba-doped system.

A first-principles study on defect association and oxygen ion migration of Sm3+ and Gd3+ co-doped ceria.

First-principles calculations based on density functional theory were performed to investigate the co-doping effects of Sm and Gd in ceria on its oxygen ion conduction. The focus of this study is on the interactions between the cation dopants and an oxygen vacancy within the two adjacent tetrahedral sites of fluorite structure surrounding the oxygen migration path. Vacancy formation energies, dopant-vacancy association energies, and migration energies were calculated to elucidate the doping effects on oxygen ion conduction. The migration energies show remarkable dependences on the ionic radii of the cations located at the edges of the migration path. A simple relation between migration energy and vacancy formation energy is proposed. This work provides an informative insight into vacancy diffusion that could be useful in optimizing doping materials for improving oxygen ion conductivity in doped ceria.