Modulated structure of the composite crystal Ca_0.83CuO₂

We have determined the crystal structure of the quasi one-dimensional cuprate Ca_0.83CuO₂, known as Ca₄Cu₅O₁₀, etc., by a superspace group approach. The compound consists of two interpenetrating subsystems of CuO₂ chains and Ca atoms. Structural parameters were refined with a superspace group of F2/m(1+α 0 γ)0s using powder X-ray and neutron diffraction data. Lattice parameters were refined to be a₁ = 2.8043(2) Å, b = 6.3179(2) Å, c₁ = 10.5744(5) Å, and β₁ = 90.10(1)° for the [CuO₂] subsystem and a₂ = 3.3652(2) Å, b = 6.3179(2) Å, c₂ = 10.5893(5) Å, and β₂ = 93.04(1)° for the [Ca] subsystem. Remarkable displacive modulation of the O and Ca atom sites is observed parallel to the b-axis and the c-axis, respectively. On the other hand, the Cu atom sites deviate mainly in the a direction to yield a periodic fluctuation between the nearest Cu-Cu distances. The Ca atoms suitably sit in the center of the modulated O₆ octahedra.