Modulated Structure of the Thermoelectric Compound [Ca_2CoO_3]_<0.62>CoO_2

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We have determined the crystal structure of the composite crystal [Ca_2CoO_3]_<0.62>CoO_2,known as Ca_3Co_4O_9,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 CoO_2 sheet and a distorted triple-layered NaCl-type Ca_2CoO_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_2CoO_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_2CoO_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_2CoO_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.
社団法人日本物理学会の論文
2002-02-15