Modulated structure of the composite crystal [2CaOH]kCoO2 (k=0.576)

We have studied the crystal structure of the new misfit-layer calcium cobaltite [2CaOH]kCoO2 (k=0.576) by means of superspace-group formalism using synchrotron x-ray diffraction data. This compound is a kind of a composite crystal which consists of two interpenetrating subsystems: (a) [CoO2]∞ containing triangular lattices formed by edge-sharing CoO6 octahedra and (b) [2CaOH]∞ double-layered rock-salt-type slabs. The two subsystems have incommensurate periodicity along the a-axis (a1/a2=k=0.576), resulting in a modulated crystal structure due to the inter-subsystem interaction. It was found that the significant structural modulation is realized in the [2CaOH] subsystem. Modulation in the [CoO2] subsystem is less marked than in the [2CaOH] ones owing to tight bonding of the closed-pack CoO6 octahedra. Thick CoO6 octahedra indicate insufficient hole-doping level in this compound. (author)