STATIC DISORDER IN THE INCOMMENSURATE STRUCTURE OF THE HIGH T-C SUPERCONDUCTOR BI(2)SR(2)CACU(2)O8+DELTA

The incommensurate modulated structure of the high T-c superconductor of nominal composition Bi2Sr2CaCu2O8 has been refined using an optimiz ed data set of 1530 reflections (496 main reflections, 753 first-order and 281 second-order satellites) in superspace group Bbmb(0 beta 1) u sing a Fourier expansion of the modulation functions up to second orde r. A large improvement of the R (wR) factor [global: 0.056 (0.051); ma in reflections: 0.049 (0.043); first-order satellites: 0.056 (0.058); second-order satellites: 0.078 (0.079)] is obtained by the introductio n of a static modulated disorder on the Bi site without any thermal fa ctor modulation and by considering occupational modulation functions. A detailed comparison with previous studies in modulated structure mod els or in supercell models is given, in particular for the description of the BiO layers.