Collapse of the charge disproportionation and covalency-driven insulator-metal transition in Sr3Fe2O7 under pressure

The effect of pressure on electronic properties and crystal structure of Sr3Fe2O7 was studied up to 45 GPa. Experimental methods employed were Fe-57 Mossbauer spectroscopy (MS), monochromatic synchrotron powder x-ray diffraction, optical reflectance between 0.6 and 4 eV, and electrical resistance measurements. Mossbauer spectra of the magnetically ordered as well as of the paramagnetic phase demonstrate that the charge disproportionation of Fe-IV disappears at pressures between 15 and 21 GPa. The diffraction data show that the tetragonal Ruddlesden-Popper-type crystal structure (space group I4/mmm) is retained up to the highest pressure. The optical spectra reveal a continuous increase with pressure of the near-infrared oscillator strength, which indicates a pressure-driven transition from an insulating towards a metallic ground state. This is confirmed by the electrical resistance measurements which evidence a sluggish pressure-induced insulator-metal (LM) transition with a clear incipient metallic state at P approximate to 20 GPa. The changes in electronic state are not associated with any detectable anomaly in the pressure dependence of lattice parameters. The high-pressure behavior of Sr3Fe2O7 is discussed in terms of a strengthening of the covalent Fe(3d)-O(2p)-Fe(3d) interactions under pressure. Within the impurity model for the electronic structure of transition metal compounds the IM transition in Sr3Fe2O7 can be attributed to the closure of a p-p-type energy gap. The ambient- and high-pressure properties of Sr3Fe2O7 and related Fe-IV oxides are compared. [S0163-1829(99)10531-9].