Analytical electron microscopic study of high-T_c superconductor Bi-Ca-Sr-Cu-O

Superconductors Bi-Ca-Sr-Cu-O containing high-T_c (110 K) and/or low-T_c (80 K) phases were investigated by analytical electron microscopy. In the sample containing the high-T_c phase, various stacking sequences formed with 2, 3 and 4 perovskite layers were observed by a lattice imaging technique. From EDX analysis, a considerable variation of composition with a higher Cu atomic fraction than that of low-T_c phase was detected. The structure of 3 perovskite layers is considered to be responsible for a superconducting transition at 110 K. A discrepancy between a volume fraction of the structure containing 3 perovskite layers and that of high-T_c phase expected from the magnetization experiment is discussed. Despite the differences of chemical composition and stacking period along the c-axis, a similar structural modulation with an incommensurate period along the b-axis was equally found in the high-T_c phase as the low-T_c phase.