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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 contentious2,3 because twinning of crystal domains hampers diffraction studies of the structure4. Recent powder diffraction refinements5,6,7 and resonant X-ray studies8,9,10,11,12 have led to proposals of a variety of charge-ordered and bond-dimerized ground-state models13,14,15,16,17,18,19. 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.

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Figure 1: Reciprocal lattice synchrotron X-ray diffraction intensities for magnetite microcrystal grains at 90 K.
Figure 2: The 168 displacement amplitudes of the low-temperature Cc magnetite structure.
Figure 3: Local distortion distributions that reveal electronic order in the low-temperature magnetite structure.
Figure 4: Charge, orbital and trimeron order in the low-temperature magnetite structure.

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Acknowledgements

We thank C. Morrison for supplying code for electrical polarization calculations, J. Honig for provision of the magnetite sample, the Leverhulme Trust for financial support, and EPSRC and STFC for financial support and provision of continuing access to ESRF.

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J.P.A. and J.P.W. designed the study, data were collected and analysed by M.S.S. and J.P.W., and J.P.A. wrote the paper with contributions from M.S.S. and J.P.W.

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Correspondence to Jon P. Wright or J. Paul Attfield.

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The authors declare no competing financial interests.

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This file contains Supplementary Methods, Supplementary Figures 1-3 with legends, Supplementary Tables 1-7 and additional references. (PDF 960 kb)

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Senn, M., Wright, J. & Attfield, J. Charge order and three-site distortions in the Verwey structure of magnetite. Nature 481, 173–176 (2012). https://doi.org/10.1038/nature10704

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