Mitsuyuki Shizuya

Modulated structure of the composite crystal [2CaOH]kCoO2 (k = 0.576)

We have studied the crystal structure of the new misfit-layer calcium cobaltite [2CaOH] k CoO 2 ( k =0.576) by means of superspace-group formalism using synchrotron x-ray diffraction data. This compound is a kind of a composite crystal which consists of two interpenetrating subsystems: (a) [CoO 2 ] ∞ containing triangular lattices formed by edge-sharing CoO 6 octahedra and (b) [2CaOH] ∞ double-layered rock-salt-type slabs. The two subsystems have incommensurate periodicity along the a -axis ( a 1 / a 2 = k =0.576), resulting in a modulated crystal structure due to the inter-subsystem interaction. It was found that the significant structural modulation is realized in the [2CaOH] subsystem. Modulation in the [CoO 2 ] subsystem is less marked than in the [2CaOH] ones owing to tight bonding of the closed-pack CoO 6 octahedra. Thick CoO 6 octahedra indicate insufficient hole-doping level in this compound.

Magnetic and electronic properties of misfit-layered cobalt oxide (Ca1−δOH)xCoO2

The magnetic and electronic properties of non- and hole-doped samples of misfit-layered cobalt oxide (Ca1−δOH)xCoO2 were investigated using the data obtained from measurements of the magnetization, specific heat, resistivity, and Seebeck coefficient. A Curie-Weiss-like behavior was observed for all samples. In a highly doped sample, a magnetic transition at 13K and metamagnetism, due to long-range antiferromagnetic ordering, were observed. This long-range ordering coexists with a nonordered paramagnetic moment, most likely enhanced by ferromagnetic fluctuations. Large Sommerfeld constants, γ=32 and 46mJmol−1K−2, were obtained from the specific heat of the doped samples. This indicates that a large effective mass of the carriers exists in the hole-doped phase of this compound. The observation of both the finite value of γ and the semiconducting resistivity indicates that the carrier is localized by the Anderson localization mechanism. The Seebeck coefficient decreases with hole doping. In low temperature, ...

Modulated crystal structure of the composite crystal (CaOH)1.14CoO2

The crystal structure of the new misfit-layer calcium cobaltite (CaOH)1.14CoO2 was determined by means of superspace-group formalism using synchrotron X-ray diffraction data. The compound belongs to the class of incommensurate composite crystals that are isostructural to an orthorhombic form of sulfide, (PbS)1.18TiS2. 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 (a 1/a 2 = 0.576), resulting in a modulated crystal structure due to the inter-subsystem interaction. In this study, it was found that there is a significant structural modulation in the [CaOH] subsystem.