Discovery of new iron oxide Fe₇O₉ and its solid solution, (Mg,Fe²⁺)₃Fe³⁺₄O₉

Abstract

We report new high-pressure polymorphs of iron oxide Fe₇O₉ (Fe³⁺/Fe²⁺ = 4/3) and its Fe²⁺-Mg solid solution (Mg,Fe²⁺)₃Fe³⁺₄O₉ synthesized at about 24-26 GPa using a multi-anvil press. Single crystal X-ray diffraction (XRD) studies showed that the crystal structures of both Fe₇O₉ and (Mg,Fe²⁺)₃Fe³⁺₄O₉ have monoclinic C2/m space groups, that differ from any other known lattices of iron oxides. Mössbauer spectra are in agreement with the crystal structure refined from single crystal XRD. This newly found Fe₇O₉ polymorph suggests that iron oxides may have more variable mixed valence state under high-pressure condition than previously thought.