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Abstract
The structural characteristics of Bi1.84Ca2.00Co1.88Ox and Bi2.49Sr2.00Co2.22Ox 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.