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. (Ca 0.4 Sr 0.6 ) 9 Co 7 O 21, Sr 14 Co 11 O 33, Sr 21 Co 17 O 51, Ba 15 Co 13 O 39, and Ba 66 Co 59 O 177 are members of an homologous series of general formula (A 3 Co 2 O 6 ) α (A 3 Co 3 O 9 ) β . All of them are constituted by the ordered intergrowth of structural frames of the phases limiting this series: Ca 3 Co 2 O 6 and 2H−BaCoO 3. All of the members of this series are commensurate phases derived by a modulation vector k = ( a * 2H + b * 2H )/3 + m 2 c * 2H, with m being related to the numbers α and β and, consequently, giving directly the A/Co ratio. The ordered succession of blocks can unambiguously be obtained from the HREM images.
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Boulahya et al. (1999) studied this question.
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