New commensurate phases in the family (A3Co2O6)α(A3Co3O9)β (A = Ca, Sr, Ba)

New phases with one-dimensional structures in the A-Co-O system (A = Ca, Sr, Ba) have been isolated and characterized by electron diffraction and high-resolution electron microscopy. (Ca0.4Sr0.6)(9)Co7O21, Sr14Co11O33, Sr21Co17O51, Ba15Co13O39, and Ba66Co59O177 are members of an homologous series of general formula (A(3)Co(2)O(6))(alpha)(A(3)Co(3)O(9))(beta). All Of them are constituted by the ordered intergrowth of structural frames of the phases limiting this series: Ca3Co2O6 and 2H-BaCoO3. All of the members of this series are commensurate phases derived by a modulation vector k = (a*(2H) + b*(2H))/3 + m2c*(2H), with m being related to the numbers alpha and beta and, consequent;ly, giving directly the A/Co ratio. The ordered succession of blocks can unambiguously be obtained from the HREM images.