A Bi-substituted Co121 single crystal, grown by a flux growth method, is studied by electron diffraction (ED) measurement and high-resolution electron microscopy (HREM). The atomic arrangement of the Bi-substituted crystal is characterized by extended discommensurate boundaries and 3/5-type monoclinic commensurate structural units, which consist of three rock-salt (RS)-type and five CoO2 subunits. The 3/5-type units are similarly observed in a Sr-substituted system. In the HREM image taken with the electron beam parallel to the  direction, two regions, namely, the discommensurate and commensurate regions, are connected parallel to the a-c plane.
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