High Voltage Electron Microscopy Station, National Institute for Materials Science^* Materials and Structures Laboratory, Tokyo Institute of Technology, Japan^** Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, Finland^*** Advanced Nano Characterization Center, National Institute for Materials Science, Japan^**** Department of Resources and Environmental Engineering, Waseda University, Japan^*****
○Takuro Nagai^* Kei Sakai^** Maarit Karppinen^*** Toru Asaka^**** Koji Kimoto^**** Atsushi Yamazaki^***** Hisao Yamauchi^** Yoshio Matsui^****

The crystal structure of a new Sr-Co-O phase was investigated through high-resolution electron microscopy (HREM) techniques [1]. We synthesized a sample in evacuated and sealed quartz ampoules at 850 ^oC from a mixture of SrO₂ and Co₃O₄ powders with the ratio of 1:1 for Sr and Co. Electron diffraction (ED) measurement together with energy dispersive x-ray spectroscopy (EDS) analysis showed that the sample contains an unknown Sr-Co-O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co = 1. A layered structure with a regular stacking of a CdI₂-type CoO₂ sheet and a rock-salt-type Sr₂O₂ double-layered block was observed in the HREM images; the phase is the parent of the more complex misfit-layered cobalt oxides of [M_mA₂O_m+2]_qCoO₂ with the formula of [Sr₂O₂]_qCoO₂, i.e. m = 0 as shown in the figure. It was also revealed that the misfit parameter q is 0.5, namely, the two sublattices of the CoO₂ sheet and the Sr₂O₂ block coexist to form a commensurate composite structure. We proposed a structural model with monoclinic P2₁/m symmetry, which was supported by simulations of ED patterns and HREM images based on dynamical diffraction theory. [1] T. Nagai et al., J. Solid State Chem. 179, 1898 (2006).