Discommensurate Structure in [(Ca_<0.90>Sr_<0.10>)_2CoO_3]_<0.61>CoO_2(Condensed matter: structure and mechanical and thermal properties)
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The crystal structure of partly Sr-substituted Co-121, i.e., "Ca_3Co_4Co_9", 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 CoC_2 sheet and a distorted three-layered (Ca,Sr)_2CoO_3 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 CoO_2 sheet has b_1=2.8346(1)Å and the (Ca,Sr)_2CoO_3 RS-type block layer has b_2=4.6391(2)Å, respectively. On the basis of the b_1/b_2 ratio and analyzed chemical compositions, the structure analogue is formulated as [(Ca_<0.90>Sr_<0.10>)_2CoO_3]_<0.61>CoO_2. In electron diffraction patterns, there are many new satellite reflections, namely 0101^^-, 0202^^-, 02^^-04, and 01^^-03, which are not present in the end member Co-121, i.e., (Ca_2CoO_3)_<0.620>CoO_2. 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  and  directions. The basic crystal structure of the present system consists of 5-CoO_2 and 3-RS subcells with b=14Å being connected discommensuratedly parallel to the a- and b-axes. Systematic introduction of stacking faults parallel to the a-b plane is also noticed.