Modulated Structure of the Composite Crystal Ca0.83CuO2

We have determined the crystal structure of the quasi one-dimensional cuprate Ca0.83CuO2, 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) A˚, b=6.3179(2) A˚, c1=10.5744(5) A˚, and β1=90.10(1)° for the [CuO2] subsystem and a2=3.3652(2) A˚, b=6.3179(2) A˚, c2=10.5893(5) A˚, 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. [Copyright &y& Elsevier]