RIETVELD ANALYSIS OF THE MODULATED STRUCTURE IN THE SUPERCONDUCTING OXIDE BI2(SR,CA)3CU2O8+X

The modulated structure of the high-Tc superconductor Bi2(Sr,Ca)3Cu2O8+x was analyzed with a newly developed Rietveld refinement program for modulated structures. The structure refinement was made by the simultaneous refinement method using x-ray and time-of-flight neutron-diffraction data. It has a one-dimensionally modulated structure with N Bbmb/11» 1, a=5.3957(3), b=5.3971(3), c=30.649(1), and k=0.2118(1)b*. Atoms in Bi-O, Sr-O, Cu-O, and Ca layers are all greatly displaced from their average positions. In particular Bi atoms are displaced mainly along the b axis, forming Bi-condensed and Bi-dilute regions. The characteristic feature of the structure is the existence of extra oxygens, giving x=1.0, in the Bi-dilute region of the Bi-O layer. Bi has a distorted rocksalt-type oxygen coordination with four short Bi-O bonds in the Bi-condensed region and a deformed square pyramidal coordination with five Bi-O bonds in the Bi-dilute region. The apical oxygen of a CuO5 pyramid, which joins Cu to Bi, moves along with Bi, distorting the pyramid considerably. © 1990 The American Physical Society.