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 diffraction from a large single crystal of Bi2Sr2CaCu2O8+x (Bi-2212) which has an incommensurately modulated superlattice. These results are important in locating the positions of the oxygen atoms and their modulation by using four-dimensional crystallographic analysis. The four-dimensional 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 with a sharp 2 K width obtained from oxygen deficient "as-grown" crystals. Our overall weighted r-factor is 0.052 including up to second order satellites, which is better than other published results for both single crystals and powders, obtained from X-ray and neutron diffraction studies. (C) 1998 Elsevier Science B.V.