Pressure-induced metallization of the perovskite Sr₃Fe₂O₇

Electrical, magnetic and structural properties of the antiferromagnetic semiconductor Sr₃Fe₂O₇ (Fe⁴⁺, d⁴ ) were probed by resistance, Mössbauer spectroscopy (MS) and X-ray diffraction (XRD) measurements up to P ≈ 40 GPa using diamond-anvil cells. A sluggish pressure-induced insulator-metal (IM) transition is observed with a clear incipient metallic state at P ≥ 20 GPa. The Fe(IV) 3d magnetic moments remain unaltered across the transition as deduced from MS, and XRD studies show no structural symmetry change up to 40 GPa. The results are consistent with carrier delocalization due to p-p gap closure, e.g., ligand-to-ligand charge transfer that does not involve the d-states and structural symmetry changes. A mechanism to account for the IM transition in the stronlium ferrates is discussed.