HIGH-PRESSURE P-P BAND CLOSURE OF THE NEGATIVE-TRIANGLE 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 d(n)L d(n)L --> d(n) + d(n)L(2) charge fluctuations. This band gap is neithe r the d-d Mott-Hubbard nor the pd 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 inves tigated high-pressure magnetic, structural, and transport properties o f Sr2FeO4 (Fe4+), an antiferromagnetic (T-N = 60 K) insulator, using d iamond anvil cells up to P = 30 GPa. With the increasing pressure a si gnificant increase in the conductance (sigma) is observed and at P-M = 19 GPa the compound metallizes as concluded from the onset of a negat ive slope of sigma(T). Magnetic studies performed with Fe-57 Mossbauer 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. (C) 1998 Elsevier Science B.V. All rights rese rved.