High-pressure p-p band closure of the negative-delta sign Sr₂FeO₄ perovskite

Recent analyses of X-ray photoemission (XPS) spectroscopy performed in some high-valence transition metal (TM) oxide insulators have implied the existence of band gaps of the p-p type, corresponding to dⁿL + dⁿL → dⁿ + dⁿL² charge fluctuations. This band gap is neither the d-d Mott-Hubbard nor the p-d charge-transfer type common to many insulating TM compounds. Thus, it is expected that upon metallization the magnetic moments and ordering will be barely affected since the d-bands are not directly involved in the band-gap closure. We have investigated high-pressure magnetic, structural, and transport properties of Sr₂FeO₄ (Fe⁴⁺), an antiferromagnetic (T_N = 60 K) insulator, using diamond anvil cells up to P = 30 GPa. With the increasing pressure a significant increase in the conductance (σ) is observed and at P_M = 19 GPa the compound metallizes as concluded from the onset of a negative slope of σ(T). Magnetic studies performed with ⁵⁷Fe Mössbauer spectroscopy showed no evidence of a magnetic moment collapse at P_M and beyond, up to P = 30 GPa. No structural phase transition occurs up to 30 GPa. The coexistence of magnetic moments and metallic state in this oxide is clear evidence of an intra-ligand p-p gap as proposed by the XPS data analyses.