X-RAY-INVESTIGATION OF THE INCOMMENSURATE MODULATED STRUCTURE OF BI2-XFE2O9+DELTA(XSR3)

A single crystal of the Bi 2212 iron oxide Bi2+xSr3-xFe2O9+delta exhib iting an incommensurate modulation, was characterized. The modulated s tructure was refined from x-ray diffraction data including satellite r eflections up to the second order and using the four-dimensional forma lism. A substitutional modulation (Bi for Sr) occurs over the Sr sites neighbouring the BiO layers whereas the other Sr sites, between the F eO2 layers, are only occupied by strontium atoms. This leads to a bism uth-rich ferrite of chemical formula Bi2.4Sr2.6Fe2O9+delta. Different types of coordination are observed both for the iron atoms and for the strontium atoms on the substituted sites. Evidence for anomalies in t he Bi positions was found; these anomalies were located in the BiO lay ers, suggesting a possible static disorder of bismuth atoms. Moreover, this effect would be closely related to the variation of the iron coo rdination. Finally, a comparison with the related superconductor compo und Bi2Sr2CaCu2O8+delta is given.