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

Abstract

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⁺ ions¹, but the ground state of the low-temperature phase has remained contentious^2,3 because twinning of crystal domains hampers diffraction studies of the structure⁴. Recent powder diffraction refinements^5,6,7 and resonant X-ray studies^8,9,10,11,12 have led to proposals of a variety of charge-ordered and bond-dimerized ground-state models^{13,14,15,16,17,18,19}. 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 ångströms. 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 prediction¹⁷. 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.