Static disorder in the incommensurate structure of the high <i>T</i> _<i>c</i> superconductor Bi₂Sr₂CaCu₂O_8+δ

Abstract

The incommensurate modulated structure of the high T c superconductor of nominal composition Bi 2 Sr 2 CaCu 2 O 8 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 (O β 1) 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.