| Sumario: | The incommensurate modulated crystal structure of the new misfit-layer calcium cobalt oxide (CaOH)CoO 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] layers containing triangular lattices formed by edge-sharing CoO octahedra, separated from each other by [2CaOH] double-layered rock-salt-type slabs. Both the subsystems are monoclinic lattices with the unit cell parameters, a = 2.81 80(4) Å, b = 4.8938(6) Å, c = 8.810(1) Å, α = 95.75(3)°, and α(=∣q∣=a/a) = 0.57822(8), viz., a = 4.8736 Å, with Z= 2. A possible superspace group is C2/m(α10)s0-C2/m(α10) 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 [2CaOH] subsystem, whereas the modulation in the [CoO] subsystem is less than in [CaOH], due to the tight bonding of the close-packed CoO octahedra. The degree of modulation in the CoO layers, i.e., the potential modulation, is almost the same as those of other compounds of the misfit-layer cobalt oxides. Flattened CoO octahedra indicate hole doping into the CoO layers. The [2CaOH] blocks act as the charge reservoir layers, and the defect Ca ions are presumably the source of the holes.
|
|---|