Modulated structure of Bi₂Sr₂CaCu₂O_8+δ, a high-T_c superconductor with monoclinic symmetry

The structure of the bismuth strontium calcium cuprate Bi_2.09Sr_1.90Ca_1.00Cu₂O_8.22 (T_c = 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) Å, q = 0.21c* + 0.14b*]. Refinement was performed in a ninefold supercell [M_r = 902, monoclinic, mS548-0.4, Cc (No. 9), a = 37.754 (7), b = 5.4109 (8), c = 41.070 (9) Å, β = 103.58 (2)°, V = 8155 (2) ų, Z = 36, D_x = 6.61 Mg m⁻³] to wR = 0.057. As in orthorhombic Bi₂Sr₂CaCu₂O_8+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 Bi₂Sr₂CaCu₂O_8+δ.

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