Modulated Structure of the Thermoelectric Compound [Ca₂CoO₃]_0.62CoO₂

抄録

We have determined the crystal structure of the composite crystal [Ca₂CoO₃]_0.62CoO₂, 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 β-angle with a=4.8339(3) Å, c=10.8436(7) Å and β=98.14(1)^º. The b-axis lengths are refined to be b₁=2.8238(2) Å for the [CoO₂] subsystem and b₂=4.5582(2) Å 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.