Cation Deficiency in (Ba, Sr)Co1-xOyHexagonal Perovskite Related Oxides: New Members of theAn+2BBnO3n+ 3Homologous Series

The new phases (Ba0.5Sr0.5)7Co6O18, Ba8Co7O21, and Ba9Co8O12have been shown by electron diffraction and high-resolution electron microscopy to be three new members (n=5,n=6, andn=7) of the 2H hexagonal perovskite-relatedAn+2BʹBnO3n+ 3homologous series. Their modulated structures are made up of chains, running along thecdirection and separated by Ba(Sr) ions, containing a succession of one Co atom in prismatic trigonal sites andnCo atoms in octahedral coordination sharing faces. The strategy to synthesize different terms of this family is discussed in terms of the distance betweenAO3layers.