Charge order and three-site distortions in the Verwey structure of magnetite

The mineral magnetite (Fe3O4) undergoes a complex structural distortion and becomes electrically insulating at temperatures less than 125 kelvin. Verwey proposed in 1939 that this transition is driven by a charge ordering of Fe2+ and Fe3+ ions(1), but the ground state of the low-temperature phase has remained contentious(2,3) because twinning of crystal domains hampers diffraction studies of the structure(4). Recent powder diffraction refinements(5-7) and resonant X-ray studies(8-12) have led to proposals of a variety of charge-ordered and bond-dimerized ground-state models(13-19). Here we report the full low-temperature superstructure of magnetite, determined by high-energy X-ray diffraction from an almost single-domain, 40-micrometre grain, and identify the emergent order. The acentric structure is described by a superposition of 168 atomic displacement waves (frozen phonon modes), all with amplitudes of less than 0.24 angstroms. Distortions of the FeO6 octahedra show that Verwey's hypothesis is correct to a first approximation and that the charge and Fe2+ orbital order are consistent with a recent prediction(17). However, anomalous shortening of some Fe-Fe distances suggests that the localized electrons are distributed over linear three-Fe-site units, which we call 'trimerons'. The charge order and three-site distortions induce substantial off-centre atomic displacements and couple the resulting large electrical polarization to the magnetization. Trimerons may be important quasiparticles in magnetite above the Verwey transition and in other transition metal oxides.