Structural modulation in the misfit-layered cobalt oxide (CaOH)kCoO2

National Institute for Materials Science
â—‹Masaaki Isobe Mitsuko Onoda Mitsuyuki Shizuya Masahiko Tanaka Eiji Takayama-Muromachi

Recently, considerable interests have been focused on layered cobalt oxides because of their various unusual physical properties. In particular, unconventional superconductivity in Na0.35CoO21.3H2O much stimulates our intellectual curiosity, because origin of the superconductivity seems to be associated with rather unusual spin-triplet-type electron pairing. Following the current research on layered cobalt oxides, new-materials search has intensified. Recently, we succeeded in synthesizing a new misfit-layered cobalt calcium hydroxide system (CaOH)kCoO2 by utilizing high-pressure technique. We report on detailed crystal structure of the compound.
The crystal structure of (CaOH)kCoO2 was analyzed by means of the superspace-group formalism using the synchrotron x-ray diffraction data. The compound belongs to the class of incommensurate composite crystals being isostructural with transition-metal sulfides (MS)xTS2 (M=Pb, Gd or La; T=Ti, V or Cr; x=1.18-1.27). The structure of the present compound consists of two interpenetrating subsystems: (i) [CoO2] containing triangular lattices formed by edge-sharing [CoO6] octahedra and (ii) [CaOH] double-layered rock-salt-type slabs. The two subsystems have incommensurate periodicity along the a-axis, resulting in modulated crystal structure due to the inter-subcell interaction. We found significant structural modulation realized in the [CaOH] subsystem.