The orthorhombic (Ba8Co6)18)α(Ba8Co8O24)β 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 diffraction and high-resolution electron microscopy. All of them exhibit orthorhombic unit cells and can be considered as members of a new homologous series (Ba8Co6O18)(alpha)(Ba8Co8O24)(beta). Their structures can be derived from the hcp stacking sequence of Ba8Co2O18 and Ba8O24 layers. Such a description 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 stabilized 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.