Structure-Controlled Oxygen Concentration in Fe₂O₃ and FeO₂

Area of Science:

• Solid-state chemistry and materials science
• High-pressure geophysics and geochemistry

Background:

• The Fe-O binary system is crucial for chemistry, materials science, and earth sciences.
• Recent synthesis of pyrite-FeO2 revealed a novel, oxygen-rich iron oxide stoichiometry, increasing the O-Fe ratio by 33%.

Purpose of the Study:

• To elucidate the solid reaction mechanism for synthesizing pyrite-type FeO2 from Fe2O3 and O2 under high pressure.
• To understand the formation and stability of O-O bonds in the FeO2 structure.

Main Methods:

• In situ X-ray diffraction experiments at high pressure.
• First-principles calculations to model phase transitions and bonding.

Main Results:

• Identified two competing phase transitions of Fe2O3 above 50 GPa: one to a post-perovskite structure (without O2) and another to pyrite-type FeO2 (with O2).
• Demonstrated that O2 diffuses into the perovskite-Fe2O3 lattice under pressure, forming O-O bonds within the pyrite-Fe2O3 structure.
• Established that these O-O bonds are kinetically stable only under high pressure.

Conclusions:

• Proposed a general mechanism for creating unconventional oxide stoichiometries by inserting oxygen into existing oxide lattices under pressure.
• Provided insights into the enrichment of oxygen in mantle minerals under high-pressure conditions.