Charge ordering transition in iron oxide Fe4O5 involving competing dimer and trimer formation

S. V. Ovsyannikov, M. Bykov, E. Bykova, D. P. Kozlenko, A. A. Tsirlin, A. E. Karkin, V. V. Shchennikov, S. E. Kichanov, H. Gou, A. M. Abakumov, R. Egoavil, J. Verbeeck, C. McCammon, V. Dyadkin, D. Chernyshov, S. van Smaalen, and L. S. Dubrovinsky

Nature Chemistry (2016), doi:10.1038/nchem.2478

Phase transitions that occur in materials, driven, for instance, by changes in temperature or pressure, can dramatically change their physical properties. Discovering new types of transitions and understanding their mechanisms is important not only from a fundamental perspective but also for practical applications.

Detailed low-temperature investigations of a recently discovered iron oxide, Fe4O5 crystallizing in a CaFe3O5-type crystal structure featuring linear chains of Fe ions, have revealed that below 150 K it is subjected to an unusual charge-ordering transition which involves competing dimeric and trimeric ordering within the chains of Fe ions (see Figure below). Bond-valence-sums analysis shows that the trimers are, in general, composed of one Fe2+ and two Fe3+ ions, likewise, the dimers are generally Fe2+–Fe3+ pairs with one shared electron.

This unusual charge-ordering transition is concurrent with a significant increase in electrical resistivity, and so, may be ascribed as of “metal-insulator” type. Magnetic susceptibility measurements and neutron diffraction establish the formation of a collinear antiferromagnetic order above room temperature and a spin canting at 85 K that gives rise to spontaneous magnetization. Thus, the phase transition discovered in Fe4O5 brings new perspectives on charge-ordered states in mixed-valent iron compounds and, in general, presents a new prototype of charge-ordering-related transitions.

Crystal structure of Fe4O5 projected down the c-axis at 300 K and at 4 K. The low-temperature structure demonstrates the preference to dimeric or trimeric ordering in different chains. Typical bond valence sums (BVS) of the dimers and trimers at 4 K are shown on the right side of this plot.

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