Strontium ferrates(IV): transition metal oxides at the insulator-metal borderline

Oxoferrates with iron in the high formal oxidation state of 4+ show a variety of electronic properties including behavior, metallic conductivity, and a valence disproportion of Fe4+. Here, we report investigations of Sr2FeO4 which crystallizes in the two-dimensional K2NiF4-type structure. From resistivity and magnetic susceptibility measurements it is found that Sr2FeO4 is an antiferromagnetic semiconductor with a Neel temperature TN of about 60 K. The Moessbauer spectra of the paramagnetic phase of Sr2FeO4 reveal a single Fe4+ quadrupole doublet. Those of the ordered phase consist of a complicated magnetic hyperfine pattern with at least four inequivalent Fe4+ sites. These may arise from structural distortions and/or from a complicated spin structure. External pressure above 6 GPa lead to an increase in near-infrared oscillator strength indicating a gap-narrowing and possibly an insulator-metal transition. Raman spectra between 20 and 300 K show, in addition to the normal Raman-active phonon modes of the K2NiF4-type crystal structure, a further oxygen-derived phonon mode. The additional Raman band vanishes near 5.5 GPa. The electronic behavior of strontium ferrates(IV) is discussed within the general systematics for the electronic structure of transition metal compounds. (orig.)