Rietveld analysis of the modulated structure in the superconducting oxide Bi₂(Sr,Ca)₃Cu₂O_8+x

The modulated structure of the high-T_c superconductor Bi₂(Sr,Ca)₃Cu₂O_8+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) Å, 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.