High-resolution electron microscopy study of misfit-layered Bi-based cobaltites

Abstract

The structural characteristics of Bi_1.84Ca_2.00Co_1.88O_x and Bi_2.49Sr_2.00Co_2.22O_x are studied by electron diffraction measurements and high-resolution electron microscopy. The crystal structures consist of two interpenetrating subsystems of a CdI₂-type CoO₂ sheet and a distorted four-layered rock-salt (RS)-type block. Both subsystems have common lattice constants a and c and a β angle. The crystal structure is incommensurately modulated parallel to the b-axis. These compounds have modulation vectors, q₁ = −a* + 0.58 and q₂ = 0.18 + 0.5c* for the Ca system, and q₁ = −a* + 0.54 and q₂ = 0.05 for the Sr system. 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 is related to the shear deformation in the a–b plane.

Acknowledgements

This study was partly supported by the Core Research for Evolution Science and Technology (CREST) Project of Japan Science and Technology Agency (JST) and also by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.