THE 4-DIMENSIONAL, INCOMMENSURATELY MODULATED, COMPOSITE CRYSTAL-STRUCTURE OF (BI, SR, CA)10CU17O29 AT 292 AND 20-K REFINED INCLUDING SATELLITE REFLECTIONS

The compound M10 Cu17 O29 with M = Bi0.06 Sr0.39 Ca0.55 has been shown to have a composite structure consisting of two interpenetrating, mod ulated sublattices. One substructure is a sandwich. Adjacent layers ar e Cu2O3 sheets extending in the ac-plane, and Bi, Sr and Ca atoms, whi ch are statistically distributed over one atomic site. The other subst ructure is interleaved between the layers of metal atoms and consists of CuO2 ribbons extending along the c-axis. Both substructures are ort horhombic, with the same a- and b-axis values, but different and mutua lly incommensurate c-axis values, which accounts for the modulation. F irst-order satellite reflections are observed. The structure is descri bed and refined in a four-dimensional formalism taking into account me asured first-order satellite reflections. All atoms exhibit significan t modulation amplitudes in particular along the b- and c-axes. Atoms i n the CuO2 ribbons have relatively large mean-square amplitudes along the ribbon axis, which supports a model of modulated ribbons with some phase disorder of the longitudinal modulation waves. Interatomic dist ances are calculated considering the effect of modulation.