Discommensurate Structure in [(Ca<SUB>0.90</SUB>Sr<SUB>0.10</SUB>)<SUB>2</SUB>CoO<SUB>3</SUB>]<SUB>0.61</SUB>CoO<SUB>2</SUB>

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  • Yubuta Kunio
    Institute for Materials Research, Tohoku University Core Research for Evolutional Science and Technology, Japan Science and Technology Agency
  • Huang Xiangyang
    Core Research for Evolutional Science and Technology, Japan Science and Technology Agency Department of Applied Physics, Graduate School of Engineering, Tohoku University
  • Miyazaki Yuzuru
    Core Research for Evolutional Science and Technology, Japan Science and Technology Agency Department of Applied Physics, Graduate School of Engineering, Tohoku University
  • Kajitani Tsuyoshi
    Core Research for Evolutional Science and Technology, Japan Science and Technology Agency Department of Applied Physics, Graduate School of Engineering, Tohoku University

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  • Discommensurate structure in 〔(Ca0.90Sr0.10)2CoO3〕0.61CoO2
  • Discommensurate structure in Ca0 90Sr0 10 2CoO3 0 61CoO2

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Abstract

The crystal structure of partly Sr-substituted Co-121, i.e., “Ca3Co4Co9”, single crystal is studied by means of the electron diffraction measurements and high-resolution transmission electron microscopy (HREM). The crystal structure consists of two interpenetrating subsystems of a CoO2 sheet and a distorted three-layered (Ca,Sr)2CoO3 rock-salt (RS)-type block layers, which belong to a triclinic system. Both subsystems have common a-, c-axes and α-, β-, γ-angle with a=4.8267(1) Å, c=10.9214(3) Å, α=90.916(2)°, β=98.291(5)°, and γ=89.957(3)°. On the other hand, the crystal structure is modulated parallel to the b-axis, along which the CoO2 sheet has b1=2.8346(1) Å and the (Ca,Sr)2CoO3 RS-type block layer has b2=4.6391(2) Å, respectively. On the basis of the b1⁄b2 ratio and analyzed chemical compositions, the structure analogue is formulated as [(Ca0.90Sr0.10)2CoO3]0.61CoO2. In electron diffraction patterns, there are many new satellite reflections, namely 010\\bar1, 020\\bar2, 0\\bar204, and 0\\bar103, which are not present in the end member Co-121, i.e., (Ca2CoO3)0.620CoO2. Because of the new superlattice reflections, the crystal structure of the Sr-doped compound is different from the end member. HREM images were taken with the incident electron beam parallel to the [0010] and [1000] directions. The basic crystal structure of the present system consists of 5-CoO2 and 3-RS subcells with b=14 Å being connected discommensuratedly parallel to the a- and b-axes. Systematic introduction of stacking faults parallel to the a–b plane is also noticed.

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