Density Functional Theory Study of Local Environment Effects on Oxygen Vacancy Properties in Magnetite

Density functional theory is employed to compute the properties of oxygen vacancies in the low-temperature monoclinic phase of magnetite (Fe3O4). A focus is placed on characterizing how the different local arrangements of Fe2+ and Fe3+ cations around the oxygen sites influence vacancy formation energies, stable defect configurations, and their dependence on the charge state and spin configuration, as well as how the vacancy induces changes in the surrounding Fe spin and charge states. We find qualitative differences in the preferred defect configurations for local environments that contain one or more nearest-neighbor Fe3+ cations on the octahedral sublattice, versus those that do not. The associated variations in the lowest-energy defect formation energies with the local environment are on the order of 0.2 eV and can vary by an additional similar to 1.1 eV for a particular local environment depending on the spin configuration. Furthermore, we present calculations of the relative energies of different models for charge order in the cubic phase, from which we argue that there is likely to be significant short-range order above the Verwey transition, such that the results presented here for the long-range-ordered monoclinic phase are expected to be also relevant for the cubic phase at low to intermediate temperatures. The implications of our results for oxygen transport are discussed.