Acta Crystallographica Section B

Structural Science

Volume 53, Part 1 (February 1997)

research papers

Acta Cryst. (1997). B53, 125-134 [ doi:10.1107/S0108768196011664 ]

Importance of True Satellite Reflections in the Analysis of Modulated, Composite Crystal Structures. II. The Structure of [M'₂Cu₂O₃]₇[CuO₂]₁₀, M' = Bi_0.04Sr_0.96

A. F. Jensen, V. Petrícek, F. K. Larsen and E. M. McCarron III

Abstract: The structure of the modulated, composite crystal bismuth strontium cuprate [M'₂Cu₂O₃]₇[CuO₂]₁₀, with M' = Bi_0.04Sr_0.96, has been determined and refined using a model which allows modulation of the atomic positional parameters for all atoms, as well as modulation of thermal parameters for the M' site. The structure is orthorhombic, with c being the misfit direction and a = 11.4712 (6), b = 13.3765 (13) Å. Sublattice 1, of composition (CuO₂)4, has c_l = 2.7509 (10) Å, while sublattice 2, of composition (M'₂Cu₂O₃)₄, has c₂ = 3.9316 (3) Å. Superspace group Amma(001 + [gamma] )ss $\overline 1$ ; q = [gamma] c_l* with [gamma] = 0.6997 (3); [lambda] = 0.6565 Å, [mu] = 25.4 mm^-1. The final residual is R_all = 0.0474 for all 1174 reflections, R_main = 0.0451 for the 549 main reflections, R_1.sat = 0.0562 for the 442 first-order satellite reflections and R_2.sat = 0.1044 for the 183 second-order satellite reflections. The satellite reflections have been collected with X-ray synchrotron radiation. It is shown that these reflections help to unambiguously determine the superspace group. The orthorhombic compound has a layered structure, stacked along the b axis. The first sublattice consists of layers of CuO₂ ribbons, with CuO₄ squares sharing edges along the c axis. The second sublattice consists of CuO₄ squares, which are sharing edges in a zigzag pattern. In-between these layers the disordered layers of Bi and Sr atoms are sandwiched. A comparison with structurally and chemically related compounds is made.