Structure of the Monoclinic-Form Misfit-Layer Compound,(Ca_(0.85)OH)_(2α)CoO_2 (a≈0.57822)

摘要

The incommensurate modulated crystal structure of the new misfit-layer calcium cobalt oxide (Ca_(0.85)OH)_(2α)CoO_2 ( was investigated using a superspace-group formalism with synchrotron X-ray diffraction data. The compound is a kind of composite crystal that consists of two interpenetrating subsystems, [CoO_2]_∞layers containing triangular lattices formed by edge-sharing CoO_6 octahedra, separated from each other by [2Ca_(0.85)OH]_∞ double-layered rock-salt-type slabs. Both the subsystems are monoclinic lattices with the unit cell parameters, a_1 = 2.8180(4) A, b = 4.8938(6) A, c = 8.810(1) A, α_0 = 95.75(3)°, and α(=|q|=a_1/ a_2) = 0.57822(8), viz., a_2 = 4.8736 A, with Z= 2. A possible superspace group is C2/m(α10)s0-C2_1/ m(a~(-1)10) for the respective subsystems. The atomic positions deviate from the average positions of the fundamental structure due to the incommensurable periodic interaction between the subsystems. A significant structural modulation was found in the [2Ca_(0.85)OH] subsystem, whereas the modulation in the [CoO_2] subsystem is less than in [2Ca_(0.85)OH], due to the tight bonding of the close-packed CoO_6 octahedra. The degree of modulation in the CoO_2 layers, i.e., the potential modulation, is almost the same as those of other compounds of the misfit-layer cobalt oxides. Flattened CoO_6 octahedra indicate hole doping into the CoO_2 layers. The [2Ca_(0.85)OH] blocks act as the charge reservoir layers, and the defect Ca ions are presumably the source of the holes.