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+alpha 0 gamma)0s using powder X-ray and neutron diffraction data. Lattice parameters were refined to be a(1) = 2.8043(2) Angstrom, b = 6.3179(2) Angstrom, C-1 = 10.5744(5) Angstrom, and beta(1) = 90.10(1)degrees for the [CuO2] subsystem and a(2) = 3.3652(2) Angstrom, b = 6.3179(2) Angstrom, c(2) = 10.5893(5) Angstrom, and beta(2) = 93.04(1)degrees 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 06 octahedra. (C) 2002 Elsevier Science.