The crystal structure of partly Sr-substituted Co-121, i.e., "Ca 3Co4Co9", 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)A° and the (Ca,Sr)2CoO3 RS-type block layer has b2 = 4.6391(2)Å, respectively. On the basis of the b 1/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 0101, 0202, 0204, and 0103, 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  and  directions. The basic crystal structure of the present system consists of 5-CoO2 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.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)