Modulated structure of the thermoelectric compound [Ca2CoO3]0.62CoO2

We have determined the crystal structure of the composite crystal [Ca2CoO3](0.62)CoO2, known as Ca3Co4O9, by a superspace group approach. Structural parameters were refined with a superspace group of Cm (0 1 - p 0) using powder X-ray and neutron diffraction data. The compound consists of two interpenetrating subsystems of a CoO2 sheet and a distorted triple-layered NaCl-type Ca2CoO3 block, being incommensurate parallel to the b-axis. The CoO2 sheet is composed of a CdI2-type triangular Co lattice with sharing an edge of the CoO6 octahedra. In the Ca2CoO3 subsystem, "<"-shaped buckled arrangement of the atoms is found, due to the ordered arrangement of the Ca and Co atoms parallel to the c-axis. Both the subsystems have common a-, c-axes and beta-angle with a=4.8339(3)Angstrom, c = 10. 8436(7)Angstrom and beta = 98.14(1)degrees. The b-axis lengths are refined to be b(1) = 2.8238(2)Angstrom for the [CoO2] subsystem and b(2) = 4.5582(2)Angstrom for the [Ca2CoO3] subsystem. An undulating arrangement of the O atom sites in the CoO2 sheets is revealed but the modulation of the Co atom site is less marked. Remarkable displacive modulation of the Co and O sites in the [Ca2CoO3] subsystem has also been observed, suggesting that a regular NaCl-type coordination is very severe for the Co-O bonds in the present compound.