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Discovery of Fe 7 O 9 : a new iron oxide with a complex monoclinic structure

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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.

No MeSH data available.


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.
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f4: 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.

Mentions: 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 Fe1-xO wüstite with the rocksalt structure transforms to a rhombohedral lattice above 20 GPa1, while at high temperatures the rocksalt structure of Fe1-xO is stable to at least 60 GPa12. High temperature-assisted phase transitions in Fe3O4 from cubic spinel to an orthorhombic phase3435, and from corundum-type α-Fe2O3 to an orthorhombic Rh2O3(II)-type or to other phases45678 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 Fe1-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 Fe3O43435 (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.


Discovery of Fe 7 O 9 : a new iron oxide with a complex monoclinic structure
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.
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f4: 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.
Mentions: 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 Fe1-xO wüstite with the rocksalt structure transforms to a rhombohedral lattice above 20 GPa1, while at high temperatures the rocksalt structure of Fe1-xO is stable to at least 60 GPa12. High temperature-assisted phase transitions in Fe3O4 from cubic spinel to an orthorhombic phase3435, and from corundum-type α-Fe2O3 to an orthorhombic Rh2O3(II)-type or to other phases45678 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 Fe1-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 Fe3O43435 (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.

View Article: PubMed Central - PubMed

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.

No MeSH data available.