HREM study of the BaCoO3-y system: Evidence for a new 5H phase

Using selected area electron diffraction (SAED) and high resolution electron microscopy (HREM) results, a structural model is proposed for a new perovskite-like material in the BaCoO3-y system: the 5H BaCoO2.74 polytype. According to X-ray diffraction data unit cell parameters are a 0.566(2) and c = 1.197(4) mm (space group <P(3)over bar m1>). The structure is based on a 5-layer stacking sequence...ccchh..., where two-fifths of the cobalt atoms are tetrahedrally coordinated by oxygen due to the presence of a cubic layer with a BaO2 composition. The accommodation of anionic vacancies within the three remaining octahedral layers is discussed. (C) 1995 Academic Press, Inc.