Role of atomic modulation in the incommensurate composite crystal structure of (Sr₂Cu₂O₃)_0.70CuO₂, "sr₁₄Cu₂₄O₄₁" with two-legged ladder and one-dimensional chain

Role of atomic modulation in the incommensurate composite crystal structure of (Sr₂Cu₂O₃)_0.70CuO₂, "Sr₁₄Cu₂₄O₄₁" has been investigated by four-dimensional superspace group approach. It has become apparent that the displacive modulations of Sr and O atoms are effective for the stabilization of alternate stacking of the planes of the Cu₂O₃ ladder and the CuO₂ chain. The Cu-O bonds in both of the modulated and unmodulated structures have been investigated to understand the origin of electronic and magnetic properties. In the average structure, it has been clarified that considerable amount of holes are generated in the CuO₂ chain. In the course of the modulated structure analysis, the Cu-O bond in the CuO₂ chain has become tilt toward the Cu₂O₃ ladder in order that the O atom in the chain acts as apical oxygen for the ladder plane. By the large modulation of the O atom in the CuO₂ chain, the prepared holes are distributed to make the hole-ordered structure in the CuO₂ plane.