Abstract
The crystal structure of our newly discovered Sr–Co–O phase is investigated in detail through high-resolution electron microscopy (HREM) techniques. Electron diffraction (ED) measurement together with energy dispersive X-ray spectroscopy (EDS) analysis show that an ampoule-synthesized sample contains an unknown Sr–Co–O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co=1. From HREM images a layered structure with a regular stacking of a CdI 2-type CoO 2 sheet and a rock-salt-type Sr 2O 2 double-layered block is observed, which confirms that the phase is the parent of the more complex “misfit-layered (ML)” cobalt oxides of [ M mA 2O m +2] q CoO 2 with the formula of [Sr 2O 2] q CoO 2, i.e. m = 0 . It is revealed that the misfit parameter q is 0.5, i.e. the two sublattices of the CoO 2 sheet and the Sr 2O 2 block coexist to form a commensurate composite structure. We propose a structural model with monoclinic P2 1/ m symmetry, which is supported by simulations of ED patterns and HREM images based on dynamical diffraction theory.
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