National Institute for Materials Science
○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 Na_0.35CoO₂1.3H₂O 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.