research papers
The incommensurate modulated structure of the high Tc superconductor of nominal composition Bi2Sr2CaCu2O8 has been refined using an optimized data set of 1530 reflections (496 main reflections, 753 first-order and 281 second-order satellites) in superspace group Bbmb(Oβ1) using a Fourier expansion of the modulation functions up to second order. A large improvement of the R (wR) factor [global: 0.056 (0.051); main reflections: 0.049 (0.043); first-order satellites: 0.056 (0.058); second-order satellites: 0.078 (0.079)] is obtained by the introduction of a static modulated disorder on the Bi site without any thermal factor modulation and by considering occupational modulation functions. A detailed comparison with previous studies in modulated structure models or in supercell models is given, in particular for the description of the BiO layers.