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The modulated structure of the high-T c superconductor Bi 2 (Sr, Ca) 3 Cu 2 O 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 …