Static disorder in the incommensurate structure of the high T c superconductor Bi2Sr2CaCu2O8+δ

Abstract

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.