Comparative study of the electronic structures of Fe3O4 and Fe2SiO4

Abstract

The electronic properties of two spinels Fe${}_3$O${}_4$ and Fe${}_2$SiO${}_4$ are studied by the density functional theory. The local Coulomb repulsion $U$ and the Hund's exchange $J$ between the $3d$ electrons on iron are included. For $U=0$, both spinels are half-metals, with the minority $t_{2g}$ states at the Fermi level. Magnetite remains a metal in a cubic phase even at large values of $U$. The metal-insulator transition is induced by the $X_3$ phonon, which lowers the total energy and stabilizes the charge-orbital ordering. Fe${}_2$SiO${}_4$ transforms to a Mott insulating state for $U>2$ eV with a gap ${\Delta }_g\sim U$. The antiferromagnetic interactions induce the tetragonal distortion, which releases the geometrical frustration and stabilizes the long-range order. The differences of electronic structures in the high-symmetry cubic phases and the distorted low-symmetry phases of both spinels are discussed.

6 pages, 6 figures