Comparative study of the electronic structures of Fe3O4 and Fe2SiO4

TL;DR

This study uses density functional theory to compare the electronic structures of Fe3O4 and Fe2SiO4 spinels, revealing how electron interactions and phonons influence their metal-insulator transitions and magnetic properties.

Contribution

It provides a detailed analysis of how Coulomb interactions and lattice distortions affect the electronic phases of Fe3O4 and Fe2SiO4, including the stabilization of charge-orbital order and Mott insulating states.

Findings

Fe3O4 remains metallic even at high U values.

Fe2SiO4 becomes a Mott insulator for U > 2 eV.

Phonon-induced distortions drive metal-insulator transitions.

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.