The orthorhombic (Ba8Co6)(18))(alpha)(Ba8Co8O24)(beta) series, a new family of monodimensional oxides

New forms of Ba-Co oxides with one-dimensional structures related to the 2H hexagonal perovskite have been synthesized and characterized by electron d iffraction and high-resolution electron microscopy. All of them exhibit ort horhombic unit cells and can be considered as members of a new homologous s eries (Ba8Co6O18)(alpha)(Ba8Co8O24)(beta). Their structures can be derived from the hcp stacking sequence of Ba8Co2O18 and Ba8O24 layers. Such a descr iption allows very close structural relationships between this orthorhombic series and the rhombohedral (Ba3Co2O6)(alpha)(Ba3Co3O9)(beta) family, also formed by a hcp stacking of mixed layers, but with different symmetry and stoichiometry, Ba3CoO6 and Ba3O9 layers. The ability of cobalt to be stabil ized in all these different types of layers allows phases belonging to both rhombohedral and orthorhombic families to be isolated thus, leading to the presence of polymorphism, for some compositions.