New type of charge-ordering transition in the novel iron oxide Fe4O5

At ambient pressure, only three iron oxide polymophs were known, FeO, Fe₃O₄, and Fe₂O₃. Recently, several new iron oxide polymorphs with hitherto unknown stoichiometries have been discovered under high pressure conditions [1,2]. These observations suggest the existence of a whole new class of iron oxide polymorphs yet to be discovered. Among them, Fe₄O₅ looks particularly exciting as it can be readily quenched at ambient pressure [1] and persists at ambient conditions as a metastable polymorph, thereby permitting detailed investigations of its physical properties and promising industrial applications.

We have performed a series of low-temperature studies using multiple probes on a recently-discovered high-pressure polymorph of iron oxide, Fe₄O₅ [1]. Fe₄O₅ has exhibited a new type of charge-ordering transition involving the formation of competing dimeric and trimeric ordering within the chains of Fe ions as revealed by X-ray studies at beamline BM01 (SNBL). These observations were also supported in neutron diffraction experiments and in measurements of magnetic and transport properties. To date, such exotic and puzzling transitions have never been documented for any other materials. 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.

Under ambient conditions, Fe₄O₅ crystallises in a CaFe₃O₅-type crystal structure featuring linear chains of octahedrally-coordinated iron ions occupying two slightly different crystallographic positions, and linear chains of FeO₆ trigonal prisms along the a-axis (Figure 1a,c). The compound contains equal amounts of Fe²⁺ and Fe³⁺ ions, and like another mixed-valent iron oxide, magnetite (Fe₃O₄), it is expected to be a good electrical conductor owing to a charge transfer between Fe²⁺ (charges) and Fe³⁺ (vacancies). Analysis of the bond valence sums in Fe₄O₅ suggested that octahedrally-coordinated iron ions have average charges of +2.76 and +2.66, whereas the cations in the prisms have an average charge of +1.92. In other words, the prisms are occupied by ferrous iron, while, the iron ions in the octahedral positions are in the mixed valence states.

Figure 1. (a, b) Examples of reciprocal lattices of X-ray diffraction intensities of Fe4O5 at 260 K and 100 K. a* and b* are the axes of the reciprocal lattices. (c) Crystal structure projected down the a-axis at room temperature. (d) Crystal structure projected along the c-axis at room and low temperatures. The low-temperature structure demonstrates the preference for dimeric or trimeric ordering in different Fe chains.

Magnetite is known to undergo a charge-ordering phase transition below ~125 K at which point its electrical resistivity abruptly jumps by about two orders of magnitude [3]. This transition in magnetite had been discovered by Verwey in 1939 [3], but only recently, after more than 70 years, the elusive charge-ordering pattern in the low-temperature phase of Fe₃O₄ has finally been uncovered by means of single-crystal X-ray diffraction [4], and the charge ordering in magnetite was found to involve ‘three-site-distortions’, called ‘trimerons’ [4]. One could expect that Fe₄O₅ could also undergo some charge ordering at relatively low temperatures.

The high-quality single crystal X-ray diffraction data collected from Fe₄O₅ at beamline BM01 unambiguously demonstrated the appearance of superlattice reflections upon cooling below 150 K (Figure 1a,b). Indexing these superlattice reflections revealed that this structure is incommensurately modulated. The main features of this transition in Fe₄O₅ are the Fe dimers and trimers within the chains of the octahedrally-coordinated cations along the a-axis (Figure 1d). Furthermore, the chains of ferrous iron are only weakly modulated along the c-axis. A constant Fe²⁺-Fe²⁺ distance (~2.861 Å at 4 K) in the low-temperature structure served as a reference point, highlighting the dramatic shortening of some interoctahedra distances (marked by elongated ellipsoids in Figure 1d). Each chain of the octahedral cations contains either dimers or trimers, which is sketched in Figure 1d. The analysis of bond-valence-sums suggested that the trimers are  composed of one Fe²⁺ and two Fe³⁺ ions, similar to the trimerons in Fe₃O₄ [3]. The Fe ions in the dimers have an oxidation state close to +2.5, i.e., the dimers are Fe²⁺–Fe³⁺ pairs with one shared electron.

This unusual charge-ordering transition in Fe₄O₅ is concurrent with a significant increase in electrical resistivity (Figure 2), and therefore it may be classified as “metal-insulator” type. In addition, the 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 magnetisation.

Figure 2. Temperature dependencies of electrical resistivity of Fe4O5 under 0 and 12 T magnetic fields (blue and green curves, respectively). These curves exhibit a bend at 150 K (marked by the arrow), indicating a ‘metal-insulator’-type transition.

Principal publication and authors
Charge ordering transition in iron oxide Fe4O5 involving competing dimer and trimer formation, S.V. Ovsyannikov (a), M. Bykov (a,b), E. Bykova (a,b), D.P. Kozlenko (c), A.A. Tsirlin (d,e), A.E. Karkin (f), V.V. Shchennikov (f,g), S.E. Kichanov (c), H. Gou (a,h), A.M. Abakumov (i,j), R. Egoavil (i), J. Verbeeck (i), C. McCammon (a), V. Dyadkin (k), D. Chernyshov (k), S. van Smaalen (b), and L.S. Dubrovinsky (a), Nature Chemistry 8, 501–508(2016); doi: 10.1038/nchem.2478.
(a) Bayerisches Geoinstitut, Universität Bayreuth (Germany)
(b) Laboratory of Crystallography, Universität Bayreuth (Germany)
(c) Frank Laboratory of Neutron Physics, JINR, Dubna (Russia)
(d) National Institute of Chemical Physics and Biophysics, Tallinn (Estonia)
(e) Institute of Physics, University of Augsburg (Germany)
(f) Institute of Metal Physics, Russian Academy of Sciences, Yekaterinburg (Russia)
(g) Institute for Solid State Chemistry, Russian Academy of Sciences, Yekaterinburg (Russia)
(h) Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao (China)
(i) Electron Microscopy for Materials Research (EMAT), University of Antwerp (Belgium)
(j) Department of Chemistry, Moscow State University (Russia)
(k) Swiss–Norwegian Beamlines at the ESRF, Grenoble (France)

References
[1] B. Lavina, et al. Proc Nat. Acad. Sci. US 108, 17281 (2011).
[2] E. Bykova, et al., Nature Commun. 7, 10661 (2016).
[3] E.J.W. Verwey, Nature 144, 327 (1939).
[4] M.S. Senn, et al. Nature 481, 173 (2012).

Top image: Crystal structure of Fe4O5 at low temperature showing dimeric and trimeric ordering.