Ab initio Investigations of Fe2+/Fe3+ Bond Dimerization and Ferroelectricity Induced by Intermediate Site/Bond-Centered Charge Ordering in Magnetite

Based on ab initio density functional calculations, we have investigated the validity of the recently proposed "intermediate site/bond-centered charge ordering'' in insulating magnetite, Fe3O4. It is found that, although the FeB2+/FeB3+ bond dimerization along the monoclinic b direction leads to local electric dipole moments, the latter are arranged in an antiferroelectric fashion in the P2/c and Cc crystal structures. Our results, based on the generalized gradient approximation plus Hubbard U (GGA+U) and Berry phase methods, clearly indicate that for the P2/c crystal structure the FeB2+/FeB3+ bond dimerization and the local electric dipole moments are suppressed at large U values. This shows that local electric dipole moments in magnetite with the P2/c symmetry are induced by the "intermediate site/bond-centered charge ordering''.