Origin of the incommensurate modulation of the 80-K superconductor Bi2Sr2CaCu2O8.21 derived from isostructural commensurate Bi10Sr15Fe10O46

Abstract

The compound ${\mathrm{Bi}}_{10}$ ${\mathrm{Sr}}_{15}$ ${\mathrm{Fe}}_{10}$ ${\mathrm{O}}_{46}$ is shown to be isostructural with the 80-K superconductor ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{O}}_{8.21}$. The incommensurate modulation of the Cu compound is commensurate in the crystal of the Fe compound that was selected. The modulation is caused by the insertion of extra oxygen atoms in the Bi layers and results in corrugated slabs. The structure of the Bi-O layers can be described as roughly 70% of rocksalt type and 30% of oxygen-deficient perovskite type. The results explain the excess oxygen in the superconductor, the mechanism causing the corrugation, and the inelastic bending property displayed by single crystals. They also reconcile apparently discrepant results about the oxygen distribution in the bismuth layers.