Observation of Iron with Eight Coordination in Iron Trifluoride under High Pressure

The diminutive ionic radius of fluorine implies probable hypercoordination in Fe−F systems. Employing sophisticated crystal structure prediction and high-pressure, high-temperature experiments, a stable orthorhombic phase of FeF₃ featuring rare 8-coordinated Fe is revealed under extreme conditions. This discovery advances comprehension of iron-based structure and contributes to exploring chemical hypercoordination phenomena.

Abstract

The chemistry of hypercoordination has been a subject of fundamental interest, especially for understanding structures that challenge conventional wisdom. The small ionic radii of Fe ions typically result in coordination numbers of 4 or 6 in stable Fe-bearing ionic compounds. While 8-coordinated Fe has been observed in highly compressed oxides, the pursuit of hypercoordinated Fe still faces significant challenges due to the complexity of synthesizing the anticipated compound with another suitable anion. Through first-principles simulation and advanced crystal structure prediction methods, we predict that an orthorhombic phase of FeF₃ with exclusively 8-coordinated Fe is energetically stable above 18 GPa—a pressure more feasibly achieved compared to oxides. Inspired by this theoretical result, we conducted extensive experiments using a laser-heated diamond anvil cell technique to investigate the crystal structures of FeF₃ at high-pressure conditions. We successfully synthesized the predicted orthorhombic phase of FeF₃ at 46 GPa, as confirmed by in situ experimental X-ray diffraction data. This work establishes a new ionic compound featuring rare 8-coordinated Fe in a simple binary Fe-bearing system and paves the way for discovering Fe hypercoordination in similar systems.

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