Discovery of Fe₇O₉: a new iron oxide with a complex monoclinic structure

Abstract

Iron oxides are fundamentally important compounds for basic and applied sciences as well as in numerous industrial applications. In this work we report the synthesis and investigation of a new binary iron oxide with the hitherto unknown stoichiometry of Fe₇O₉. This new oxide was synthesized at high-pressure high-temperature (HP-HT) conditions, and its black single crystals were successfully recovered at ambient conditions. By means of single crystal X-ray diffraction we determined that Fe₇O₉ adopts a monoclinic C2/m lattice with the most distorted crystal structure among the binary iron oxides known to date. The synthesis of Fe₇O₉ opens a new portal to exotic iron-rich (M,Fe)₇O₉ oxides with unusual stoichiometry and distorted crystal structures. Moreover, the crystal structure and phase relations of such new iron oxide groups may provide new insight into the cycling of volatiles in the Earth’s interior.

Introduction

Iron oxides have great importance for all natural sciences as well as numerous industrial applications. Considering the high abundance of oxygen and iron in the Earth’s crust and the mantle, binary iron oxides and their derivatives are important endmembers of phases that make a significant contribution to properties of the Earth. Many studies have been devoted to investigations of various properties of iron oxides at conditions relevant to the Earth’s interior, i.e., at high pressure or at high pressure combined with high temperature (HP-HT). These studies reported a number of remarkable findings for the three basic iron oxides, Fe_1-xO wüstite^1,2, α-Fe₂O₃ hematite^3,4,5,6,7,8, and Fe₃O₄ magnetite^9,10,11. Meanwhile, several experimental and theoretical studies indicated that the chemistry of iron oxides at extreme conditions of high pressure and high temperature may extend to intriguing behavior beyond these three well-known oxides^12,13,14,15. For instance, recent experimental HP-HT investigations in the pressure range of 10–20 GPa reported two new orthorhombic Cmcm polymorphs with Fe₄O₅¹³ and Fe₅O₆¹⁴ stoichiometry, i.e., between Fe₃O₄ and Fe_1-xO, and hence, with mixed Fe²⁺ and Fe³⁺ oxidation states. More interestingly, using modern theoretical methods an opposite tendency in the stoichiometry changes in Fe-O systems was found, suggesting an extended stability of ferric hematite and a gradual shift to Fe⁴⁺–bearing oxides at megabar pressures¹⁵. In particular, these studies predicted the existence of a new exotic FeO₂ oxide with highly charged Fe⁴⁺ ions that should have an extended range of stability against decomposition under megabar pressures¹⁵. Recently, a number of experimental studies have shown the important role of the iron oxides under HP-HT conditions in the Earth’s deep interior^16,17,18,19. Decomposition reactions of iron oxides and oxyhydroxides may induce the release of oxygen or hydrogen in the Earth’s lower mantle^17,18.

In addition to their obvious and primary importance for geosciences, iron oxides play crucial roles in many technological processes and applications, and remain among a handful of key materials with significant impact on the fundamental behavior of materials, including charge carrier transfer and interactions, spin dynamics of electrons, and other central topics. In other words, iron oxides are important prototype materials. For instance, the oldest-known natural magnet, magnetite, demonstrates a fascinating ‘metal-insulator’-type transition near 120–125 K²⁰, named later as the ‘Verwey transition’ after its discovery in 1939. This Verwey transition was believed to be related to charge ordering on octahedral sites in the spinel structure, and has been hotly debated for decades. Only recently it was revealed that this transition is linked to formation of hitherto unknown ‘quasiparticles’ consisting of three bonded Fe ions called ‘trimerons’²¹. In hematite another landmark transition was discovered near 255 K with a related abrupt and drastic reorientation of spins of Fe³⁺ ions, named afterwards as a ‘spin-flop’ or Morin transition²². Also iron-deficient wüstite, Fe_1-xO, displays puzzling complexities with regard to stoichiometry, defect structure, and elastic and physical properties^{23,24,25,26,27,28}, and serves as a prototype for systems with non-stoichiometry. Recently it was experimentally demonstrated that, similar to magnetite, the newly-discovered Fe₄O₅¹³ also undergoes an unprecedented ‘metal-insulator’–type transition upon cooling below 150 K with competing dimeric and trimeric ordering in the Fe chains, leading to strong structural modulations²⁹. Therefore, synthesis of new iron oxides with both mixed Fe²⁺ and Fe³⁺ valences and bearing highly charged Fe⁴⁺ ions is of significant interest for many scientific fields.

In the present work we investigated the phase stability of iron oxides at HP-HT conditions (see Methods) and discovered in samples recovered at ambient conditions the presence of crystals of a new iron oxide with unusual Fe₇O₉ stoichiometry. Fe₇O₉ with its ratio of Fe/O ~ 0.777 lies between Fe₃O₄ and newly-discovered Fe₄O₅¹³, but in contrast to both, it is non-magnetic at ambient conditions and adopts a monoclinic crystal structure with four sites for Fe cations. These observations combined with the difference in Fe³⁺/Fe²⁺ ratios (⁴/₃ in Fe₇O₉ versus 2 in Fe₃O₄ and 1 in Fe₄O₅) suggest that the physical properties of Fe₇O₉ may be remarkably different. In addition, we also synthesized Fe₇O₉ containing a significant amount of Mg and discuss possible geological implications for this new polymorph.

Results and Discussion

Single crystal XRD measurements

The single crystals of pure Fe₇O₉ and Mg-doped (Mg,Fe²⁺)₃Fe³⁺₄O₉ had sizes less than 50 μm in their linear dimensions that restricted detailed investigations of their properties. The chemical composition of the samples was determined using conventional electron microprobe methods and from single crystal X-ray diffraction data. We also collected Mössbauer spectra from these samples to determine the oxidation states of the Fe ions.

By means of single crystal X-ray diffraction on the crystals (Fig. 1 and Supplementary Figure 1) we solved and refined the crystal structures of Fe₇O₉ and (Mg,Fe²⁺)₃Fe³⁺₄O₉ at ambient conditions and confirmed their stoichiometry. Crystals of Fe₇O₉ and (Mg, Fe²⁺)₃Fe³⁺₄O₉ had no pronounced asymmetry in their shape, but were rather small (0.03 × 0.02 × 0.01 and 0.03 × 0.02 × 0.02 mm³), so it was not possible to perform an analytical absorption correction based on crystal shape. Crystals of Fe₇O₉ appeared to be twinned, and due to the high degree of peak overlap (>50%), we had to integrate both twin domains simultaneously to perform a refinement of the crystal structure against HKLF5 data (BASF value was about 49.8%). The twinning could also influence the quality of determination of the anisotropic parameters. For consistency, we also refined the structure in an isotropic approximation and demonstrated a negligible influence on the atomic positions (CIF-files in the Supplementary Materials). Technical details of the structure determinations are summarized in Tables S1 and S2 in the Supplementary Materials. We established that both compounds adopt the same monoclinic structure of the C2/m space group. The unit cell parameters in Fe₇O₉ are as follows: a = 9.696(2) Å, b = 2.8947(6) Å, c = 11.428(3) Å, β = 101.69(2)°, V = 314.10(12) ų, and Z = 2 (Fig. 2, and Tables S1 and S2 and CIF-files in the Supplementary Materials). The crystal structure of Fe₇O₉ has four different crystallographic sites for cations, three (Fe1, Fe2, Fe3) are octahedrally–coordinated and connected in a 3D network, while the fourth, Fe4, has a trigonal-prismatic arrangement (Fig. 2 and Table S2 in the Supplementary Materials). By analyzing the Fe-O bond distances in this polymorph using a bond valence sums (BVS) method³⁰, we established the average oxidation state for Fe ions occupying the Fe1, Fe2, Fe3, and Fe4 sites to be +2.74, +2.72, +2.82, and +2.10, respectively. Thus, we conclude that the Fe4 sites are occupied almost exclusively by Fe²⁺ ions (Fig. 2). The other octahedral sites participate in electronic exchange between Fe²⁺ and Fe³⁺ ions through the polaron hopping mechanism, similar to the octahedral network of magnetite²⁰ and other Fe-bearing oxides³¹. Thus, the BVS method shows the average charge for each of the Fe1-Fe3 sites. We note that various considerations lead to octahedral sites in the recently-discovered Fe₄O₅ phase having different charges²⁹.

Figure 1

X-ray diffraction images.

(a) Example of X-ray diffraction image of microscopic single crystal of (Mg,Fe)₃Fe₄O₉ collected at ambient conditions under rotation of a sample over 360 degrees in a beam. This image contains several hundreds of small well-resolved reflections. Insets show selected magnified areas of this image that better show indistinct reflections. (b,c) Projections of these X-ray diffraction data in reciprocal space, for two selected planes, 1kl (b) and h1l (c).

Figure 2

The crystal structure of Fe₇O₉.

This structure corresponds to ambient conditions and is shown in a projection along the b axis.

Mg-doped Fe₇O₉ crystals adopt the same crystal structure as Fe₇O₉ with similar unit cell parameters: a = 9.6901(12) Å, b = 2.8943(5) Å, c = 11.4397(15) Å, β = 102.045(14)°, V = 313.77(8) ų, and Z = 2 (Fig. 2 and Table S1 in the Supplementary Materials). Electron microprobe analysis established their chemical composition to be Mg_1.06Fe_5.94O₉, i.e., nearly 15% of Fe ions are substituted by Mg. We used this chemical composition in the crystal structure refinement and found that Mg²⁺ ions occupy all four Fe sites, but with a noticeable preference for (i) the spacious Fe4 sites that are occupied by the larger Fe²⁺ ions in Fe₇O₉, and (ii) the Fe1 sites, located between the Fe4 sites (Fig. 2, Table S2 in the Supplementary Materials). In the case of Mg doping of Fe₄O₅, Mg ions were also found to occupy all Fe sites in the structure³². Repeating the same BVS analysis³⁰ as above for Fe₇O₉ taking into account the Mg atom distribution determined by single crystal X-ray diffraction (Table S2 in the Supplementary Materials), we confirmed the ferrous nature of Fe4 ions and detected a minor increase in the BVS of all other octahedrally coordinated Fe ions as +2.81, +2.78, and +2.87 for the Fe1, Fe2, and Fe3 sites, respectively (Fig. 2). Thus, the results of BVS analysis suggest the persistence of charge transfer in the 3D octahedral network with the distributed Mg ions. However, these Mg impurities should dramatically lower the mobility of hopping polarons, and hence, the bulk electrical conductivity of Mg-doped Fe₇O₉ is expected to be much lower than that of Fe₇O₉.

Mössbauer spectroscopy

Both Fe₇O₉ and (Mg,Fe)₇O₉ samples were analyzed by Mössbauer spectroscopy using a synchrotron Mössbauer source that gave excellent signal to noise ratios (Fig. 3 and Table S3 in the Supplementary Materials). We did not observe any magnetic component in these spectra, and hence conclude that these materials are non-magnetic at ambient conditions. The spectrum of Fe₇O₉ (Fig. 3a) could be fitted by a superposition of two basic components, including (i) Fe²⁺ ions at the prismatically-coordinated Fe4 sites in the crystal structure (Fig. 2) and (ii) a merged component related to octahedrally-coordinated Fe1, Fe2, and Fe3 ions with an average oxidation state of Fe^2.8+ (Fig. 3a and Table S3 in the Supplementary Materials). We note that this Fe^2.8+ component is an average because of the above-mentioned rapid charge exchange between Fe²⁺ and Fe³⁺ ions at the octahedral sites, similarly to Fe₃O₄³³. This finding is in excellent agreement with the above BVS oxidation states of the Fe ions obtained from the single crystal XRD data. It should be noted here that the three slightly structurally-inequivalent Fe1, Fe2 and Fe3 sites (Fig. 2) give similar contributions to the Mössbauer spectrum because of the very similar environment (edge-sharing FeO₆ polyhedron) of the Fe ions, so it is not possible to separate their individual components in the spectrum (Fig. 3a). Spectra obtained from (Mg,Fe)₇O₉ were quite similar to Fe₇O₉ (Fig. 3b), while the merged component in the Mg-bearing sample spectrum shows a slightly higher average oxidation state of Fe^2.9+ due to incorporation of Mg.

Figure 3

Mössbauer spectra of Fe₇O₉ samples.

(a) Single crystal of Fe₇O₉. (b) Single crystal of (Mg,Fe)₃Fe₄O₉. Both spectra were collected at ambient conditions. Black open circles, experimental spectrum; lines, fitted spectra; broken line, residual.

Discussion

In our work we synthesized Fe₇O₉ and (Mg,Fe²⁺)₃Fe³⁺₄O₉ crystals at high pressures around 24–26 GPa. In previous studies the orthorhombic polymorphs of Fe₄O₅¹³ and Fe₅O₆¹⁴ were prepared at substantially lower pressures, between 10 and 20 GPa. It is interesting to note that the first pressure-driven structural phase transitions in the known iron oxides were detected at similar pressures around 20–25 GPa. For instance, at room temperature cubic Fe_1-xO wüstite with the rocksalt structure transforms to a rhombohedral lattice above 20 GPa¹, while at high temperatures the rocksalt structure of Fe_1-xO is stable to at least 60 GPa^1,2. High temperature-assisted phase transitions in Fe₃O₄ from cubic spinel to an orthorhombic phase^34,35, and from corundum-type α-Fe₂O₃ to an orthorhombic Rh₂O₃(II)-type or to other phases^4,5,6,7,8 were also observed in some studies already above 20–25 GPa, although these phase transitions are still hotly debated. These observations suggest that all conventional iron oxides (α-Fe₂O₃, Fe₃O₄, and Fe_1-xO) become unstable with respect to structural transformations in approximately similar pressure ranges that might be related to similar shortening of Fe-O bond lengths in their structures. Hence, the resultant high-pressure polymorph of a compressed and heated iron oxide could depend on its stoichiometry, thereby suggesting chemical tuning as a route to new structural phases. In general, one could expect that a minor tuning in stoichiometry could lead either to structures with vacancies (ordered or disordered) or to modified, Fe₃O₄-like or Fe₂O₃-like high-pressure phases in new oxides. Likewise, significant shifts from known stoichiometry could potentially lead to hitherto unknown structures. For instance, the newly-discovered orthorhombic Cmcm polymorphs of Fe₄O₅¹³ and Fe₅O₆¹⁴ crystalize in structures that are linked to the high-pressure orthorhombic polymorph of Fe₃O₄^34,35 (Fig. 4). By analogy with the existing family of calcium ferrites, ³⁶, it was proposed that iron oxides with this Cmcm structure could also form such a family as ³⁷, which includes Fe₃O₄, Fe₄O₅¹³ and Fe₅O₆¹⁴. However, the present discovery of Fe₇O₉ that does not belong to this family on the one hand, but having a certain structural similarity with the above oxides on the other hand (Fig. 4), suggests that the family of iron oxides that are structurally linked to the high-pressure polymorph of Fe₃O₄ may be more broad, e.g., like , thereby suggesting a potentially greater diversity than in the calcium ferrite oxides. For instance, Fe₉O₁₁ (n = 2) or Fe₁₁O₁₃ (n = 3) might be hypothetically stable under certain HP-HT conditions.

Figure 4

Comparison of unit cells of crystal structures of iron oxides.

(a) High-pressure orthorhombic polymorph of Fe₃O₄³⁴, (b) Monoclinic Fe₇O₉ polymorph discovered in the present work. (c) Orthorhombic Cmcm polymorph of Fe₄O₅ discovered in ref. 13. (d) Orthorhombic Cmcm polymorph of Fe₅O₆ discovered in ref. 14. Different colors of the octahedra denote different crystallographic sites for Fe ions.

At the moment the stability field of this new Fe₇O₉ polymorph is not well defined, although it appears to lie at pressures higher than those of Fe₄O₅¹³ and Fe₅O₆¹⁴. The chemical compositions and the structural phases of iron-magnesium oxides in the Earth’s mantle remain a disputed issue that requires in situ investigations at HP-HT conditions under different oxygen fugacities. In this regard, the unexpected discovery of a Fe₇O₉ polymorph provides new insight into possible compositions of mantle phases, and provides a new type compound that may play a key role in determining physical and chemical properties.

The new oxide, Fe₇O₉, is a compound with a ratio of Fe³⁺/Fe²⁺ intermediate between those of Fe₃O₄ and Fe₄O₅ (Fig. 4). Both Fe₃O₄ and Fe₄O₅ are model systems for investigations of Fe²⁺–Fe³⁺ interactions in solids, demonstrating enigmatic low-temperature phase transitions of ‘metal-insulator’-type that lead to the formation of exotic ‘trimeron quasiparticles’ in Fe₃O₄²¹ or to even more intricate ordering patterns in Fe₄O₅²⁹. We note that the low-temperature Verwey transition in Fe₃O₄ has had a strong impact on solid state physics and chemistry for decades. Thus, Fe₇O₉ presents an exciting compound that promises important implications for geosciences, solid state physics and chemistry with potential for industrial applications.

The recently discovered iron oxides may play important roles in the cycling of volatiles in the Earth’s deep interior^16,17,18,19. For instance, oxygen could be released in the deeper part of the lower mantle via decomposition reactions of Fe₂O₃ and Fe₃O₄ into Fe₅O₇ and Fe₂₅O₃₂ above 60 GPa¹⁷. Further, it was recently reported that FeO₂ could be formed in the lower mantle as a product of FeOOH goethite decomposition¹⁸. Moreover, this reaction could supply hydrogen to the surrounding mantle¹⁸. Physical and chemical properties of the newly-discovered iron oxides can therefore provide novel insights into the chemical evolution of the Earth’s interior.