Nonequivalence of the octahedral sites of cubicFe3O4magnetite

Abstract

We present results of electronic structure calculations for the room-temperature cubic phase of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ magnetite using $\mathrm{GGA}+U$. We use a lower-symmetry, orthorhombic structure that allows for two independent octahedral sites. For low values of the Hubbard cross-correlation parameter $U$, the two octahedral sites are equivalent, as they appear in experimental results. For values of $U\ensuremath{\gtrsim}2.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, the octahedral sites are shown to be nonequivalent in their magnetic moments, charges, electric field gradients, and densities of states. We interpret this result to indicate either that (1) commonly used values of $U$ may create an artificial nonequivalence or (2) the octahedral sites of magnetite are not equivalent for time scales shorter than the experimental interaction times.