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.
Highlights
Iron oxides have great importance for all natural sciences as well as numerous industrial applications
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 highpressure high-temperature (HP-HT) conditions and discovered in samples recovered at ambient conditions the presence of crystals of a new iron oxide with unusual Fe7O9 stoichiometry
Summary
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 Fe2.8+ (Fig. 3a and Table S3 in the Supplementary Materials) We note that this Fe2.8+ component is an average because of the above-mentioned rapid charge exchange between Fe2+ and Fe3+ ions at the octahedral sites, 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. 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 Fe2.9+ due to incorporation of Mg
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