MODULATED STRUCTURE OF THE COMPOSITE CRYSTAL CA13.6SR0.4CU24+YO41+Z

We determined the crystal structure of Ca13.6Sr0.4Cu24+yO41+z (y = -0. 017 and z = -0.035) by a technique of combined neutron and X-ray Rietv eld refinement using a superspace group approach. This oxide is a kind of a composite crystal consisting of two interpenetrating subsystems: (a) [(Cs,Sr)(2)Cu2O3](infinity) containing alkaline-earth metals and spin-1/2 Heisenberg two-leg ladders and (b) one-dimensional [CuO2](inf inity) chains of CuO4 squares sharing two opposite edges. The most rem arkable structural feature is that parts of Cu1 atoms on the ladder ar e coordinated to O3 atoms situated in the neighboring [CuO2](infinity) chains, forming CuO5 tetragonal pyramids. Cu1 atoms in the CuO5 pyram ids have shorter equatorial Cu1-O bonds than those in CuO4 squares, wh ich evidences hole doping into the 5-coordinated Cu1 atoms. Modulation s are less marked in the [Cu2O3](infinity) ladder than in the [CuO2](i nfinity) chain owing-to the presence of zigzag chains of edge-sharing CuO4 squares and corner sharing of O-2 atoms in the ladder.