A new orthorhombic Ba8Co7O21 phase:: Polymorphism in the (Ba3Co2O6)α(Ba3Co3O9)β 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 (A(3)B(2)O(6))(alpha)(A(3)B(3)O(9))(beta). 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 a(o) = 1.14, b(o) = 1.98, and c(o) = 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, (C) 2000 Academic Press.