High-Pressure Polymorphism in Silver Ferrite Delafossite, AgFeO 2 .

The delafossites are a class of layered metal oxides that are notable for being able to exhibit optical transparency alongside an in-plane electrical conductivity, making them promising platforms for the development of transparent conductive oxides. Pressure-induced polymorphism offers a direct method for altering the electrical and optical properties in this class, and although the copper delafossites have been studied extensively under pressure, the silver delafossites remain only partially studied. We report two new high-pressure polymorphs of silver ferrite delafossite, AgFeO ₂ , that are stabilized above ∼6 and ∼14 GPa. In situ X-ray diffraction and vibrational spectroscopy measurements are used to examine the structural changes across the two phase transitions. The high-pressure structure between 6 and 14 GPa is assigned as a monoclinic C 2/ c structure that is analogous to the high-pressure phase reported for AgGaO ₂ . Nuclear resonant forward scattering reveals no change in the spin state or valence state at the Fe ³⁺ site up to 15.3(5) GPa.