Importance of True Satellite Reflections in the Analysis of Modulated, Composite Crystal Structures. II. The Structure of [M'2Cu2O3]7[CuO2]10, M' = Bi0.04Sr0.96

Abstract

The structure of the modulated, composite crystal bismuth strontium cuprate [M′2Cu2O3]7[CuO2]10, with M′ = Bi0.04Sr0.96, 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 c 2 = 3.9316 (3) Å. Superspace group Amma(001 + γ)ss\overline 1; q = γc l* with γ = 0.6997 (3); λ = 0.6565 Å, μ = 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.