Modulated Structure of the Thermoelectric Compound [Ca2CoO3]0.62CoO2

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Abstract
We have determined the crystal structure of the composite crystal [Ca 2 CoO 3 ] 0.62 CoO 2 , known as Ca 3 Co 4 O 9 , by a superspace group approach. Structural parameters were refined with a superspace group of C m (0 1- p 0) using powder X-ray and neutron diffraction data. The compound consists of two interpenetrating subsystems of a CoO 2 sheet and a distorted triple-layered NaCl-type Ca 2 CoO 3 block, being incommensurate parallel to the b -axis. The CoO 2 sheet is composed of a CdI 2 -type triangular Co lattice with sharing an edge of the CoO 6 octahedra. In the Ca 2 CoO 3 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 β-angle with a =4.8339(3) Å, c =10.8436(7) Å and β=98.14(1)°. The b -axis lengths are refined to be b 1 =2.8238(2) Å for the [CoO 2 ] subsystem and b 2 =4.5582(2) Å for the [Ca 2 CoO 3 ] subsystem. An undulating arrangement of the O atom sites in the CoO 2 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 [Ca 2 CoO 3 ] subsystem has also been observed, suggesting that a regular NaCl-type coordination is very severe for the Co–O bonds in the present compound.