Abstract
The structure and magnetic properties of the () materials were studied. The limits of solid solution formation are at for the Fe system and for the Co system. Crystals of the new phase were grown and the structure established by x-ray crystallography. The subcell is the same as that of the 10-K superconductor, , but the superstructure is different, as it exhibits a commensurate modulation of periodicity 4 instead of 5. Extra oxygen is accommodated in the Bi layers, as in , and the structure of the Bi-O layers can be described as 50% rocksalt type and 50% oxygen-deficient perovskite for , but with disorder at the oxygen positions. The formal valence of Co in this compound is about 2.5±0.2 as deduced from structural and chemical measurements, whereas Fe adopts the oxidation state +3 as deduced by Mössbauer measurements. is an antiferromagnetic insulator with the spins lying within the Co sheets and the antiferromagnetic ordering temperature () is sensitive to processing conditions and composition changes. The high anisotropy of the susceptibility suggests that may be an Ising or antiferromagnet.
- Received 24 January 1989
DOI:https://doi.org/10.1103/PhysRevB.39.11587
©1989 American Physical Society