Modulated Structure of the Composite Crystal Ca0.83CuO2

Abstract We have determined the crystal structure of the quasi one-dimensional cuprate Ca 0.83 CuO 2 , known as Ca 4 Cu 5 O 10 , etc., by a superspace group approach. The compound consists of two interpenetrating subsystems of CuO 2 chains and Ca atoms. Structural parameters were refined with a superspace group of F 2/ m (1+ α 0 γ )0 s using powder X-ray and neutron diffraction data. Lattice parameters were refined to be a 1 =2.8043(2) A, b =6.3179(2) A, c 1 =10.5744(5) A, and β 1 =90.10(1)° for the [CuO 2 ] subsystem and a 2 =3.3652(2) A, b =6.3179(2) A, c 2 =10.5893(5) A, and β 2 =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 O 6 octahedra.