Modulated structure of Bi2Sr2CaCu2O8+δ, a high-Tc superconductor with monoclinic symmetry

The structure of the bismuth strontium calcium cuprate Bi2.09Sr1.90Ca1.00Cu2O8.22 (Tc = 90 K) was studied by single-crystal X-ray diffraction. A two-component modulation vector was found with respect to the C-centred orthorhombic cell of the average structure [a 5.4112 (7), b = 30.873 (7), c = 5.4161 (20) A, q = 0.21c* + 0.14b*]. Refinement was performed in a ninefold supercell [Mr = 902, monoclinic, mS548-0.4, Cc (No. 9), a = 37.754 (7), b = 5.4109 (8), c = 41.070 (9) A, β = 103.58 (2) , V = 8155 (2) A3, Z = 36, Dx = 6.61 Mg m-3] to wR = 0.057. As in orthorhombic Bi2Sr2CaCu2O8+δ, the structure of this new monoclinic variant contains perovskite-derived slabs delimited by a BiO layer on each side. All metal atoms show a transverse displacement wave which has the largest amplitude for the Cu and Ca atoms. In addition, the Bi and Sr atoms are displaced longitudinally. Bi-rich regions with a distorted rocksalt-type atom arrangement and Bi-poor regions with chains of corner-linked BiO3 Ψ-tetrahedra are formed. One additional oxygen site per translation unit of the modulation wave was detected in the Bi-poor regions of the chains, giving the composition Bi9O10. Approximately 10% of Bi substitution was found on the Ca sites, but no clear indication of preferential sites was observed. A partial substitution by Ca was refined at Sr sites situated near the regions where the additional O atoms are located. The monoclinic symmetry corresponds to a systematic shift of the modulation waves of consecutive slabs by π/9 with respect to orthorhombic Bi2Sr2CaCu2O8+δ. (orig.)