REFINEMENT OF THE INCOMMENSURATE STRUCTURE OF HIGH-QUALITY BI-2212 SINGLE-CRYSTALS FROM A NEUTRON-DIFFRACTION STUDY

We report the results of the refinement of data collected by neutron d iffraction from a large single crystal of Bi2Sr2CaCu2O8+x (Bi-2212) wh ich has an incommensurately modulated superlattice. These results are important in locating the positions of the oxygen atoms and their modu lation by using four-dimensional crystallographic analysis. The four-d imensional space group is found to be consistent with N Bbmb/1(1) over bar1$. We do not find excess oxygen but do observe large modulations in the oxygen associated with the bismuth ions in the lattice. This is consistent with the superconducting transition temperature of 91 K wi th a sharp 2 K width obtained from oxygen deficient ''as-grown'' cryst als. Our overall weighted r-factor is 0.052 including up to second ord er satellites, which is better than other published results for both s ingle crystals and powders, obtained from X-ray and neutron diffractio n studies. (C) 1998 Elsevier Science B.V.