OPTICAL REFLECTIVITY AND RAMAN-SPECTRA OF SR2FEO4 UNDER PRESSURE

We have investigated the effect of pressure on the optical reflectivity (0.5-4 eV, T=300 K) and Raman spectra (T=20-300 K) of Sr2FeO4, an insulating compound with iron in the high oxidation state +4. The lowest optical transition near 1.2 eV shifts to lower energy with increasing pressure, and the overall near-infrared oscillator strength starts to increase significantly for pressures above 6 GPa. This behavior is interpreted in terms of a gap narrowing between a local 5Eg ground state and a d5L-1-like continuum possibly followed by an insulator-metal transition. In addition to the phonon Raman modes, which are allowed in the K2NiF4-type crystal structure, we observe a broad Raman band near 350 cm-1. This band develops into a sharp feature upon cooling to 20 K and vanishes in a narrow pressure range around 5.5 GPa. Oxygen isotope substitution experiments prove that the band is related to oxygen vibrations. We consider explanations where the 350 cm-1 mode arises from spin-induced phonon scattering or from small structural displacements of Fe ions. © 1994 The American Physical Society.