A New Orthorhombic Ba8Co7O21 Phase: Polymorphism in the (Ba3Co2O6)(Ba3Co3O9) System

The systematic search of new intermediate phases between 2H-BaCoO3 and Ca3Co2O6 structures has shown, for the first time, the existence of polymorphism in monodimensional phases of the homologous series (A3B2O6)ALPHA(A3B3O9)BEAT. A new Ba8Co7O21 phase, different from the trigonal phase previously reported, has been synthesized. The new material is orthorhombic (SG Fd2d) with unit cell parameters ao=1.14, bo=1.98, and co=1.75 nm. Both orthorhombic and trigonal phases are built up of isolated rows of polyhedra parallel to the c-axis formed by one trigonal prism and six ocathedra sharing faces, but with a different spatial arrengement between the chains. The orthorhombic structure can be derived from a hcp stacking of Ba8O24 and Ba8Co2O18 layers.