Modulated Structure of Misfit-Layered Compound [Bi2.08Sr1.67Oy]0.54[CoO2]

The modulated structure of Bi 2.49 Sr 2.00 Co 2.22 O x is studied by electron diffraction analysis and high-resolution electron microscopy. The crystal structure consists of two interpenetrating subsystems of a CoO 2 sheet and a distorted four-layered rock-salt-type (Bi,Sr)O block. Both subsystems have common a -axes, c -axes and β-angles with a =4.99 Å, c =15.25 Å, and β=93.6°. On the other hand, the crystal structure is incommensurately modulated parallel to the b -axis, in which b 1 =2.78 Å for the CoO 2 sheet and b 2 =5.12 Å for the (Bi,Sr)O block, respectively. These vectors characterize the structural analogue as [Bi 2.08 Sr 1.67 O y ] 0.54 [CoO 2 ]. It is found that this compound has various modulation vectors, q 1 =-a * +0.54b 1 * , q 2 =0.05b 1 * and locally q 3 =-0.14a * +0.10b 1 * . The intrinsic modulation vector q 2 is different from that in [Bi 0.87 SrO 2 ] 2 [CoO 2 ] 1.82 studied by Leligny et al. High-resolution images taken with the incident electron beam parallel to the a - and c -axes clearly exhibit modulated atomic arrangements. The local modulation mode characterized by the vector q 3 is associated with the shear deformation in the a – b plane.