MODULATED STRUCTURES OF (BI1-XPBX)2SR2COOY EXAMINED BY HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY

The Pb-doped Bi-Sr-Co-O complex oxides, (Bi1-xPbx)2Sr2CoOy with x=0.0 to 0.5, are examined by means of electron diffraction and high-resolution transmission electron microscopy (HRTEM) in order to clarify the effects of Pb-doping on the commensurate modulated structure of Bi2Sr2CoOy. Specimens with Pb content up to 10% (x less-than-or-equal-to 0.1 ) have a commensurate modulated structure with modulation periodicity b(s)=4b. At 15% substitution (x=0.15), a stepwise increase of the value of b(s) is observed and the specimens with Pb content from 20 to 30% (0.2 less-than-or-equal-to x less-than-or-equal-to 0.3) have incommensurate modulations with b(s) almost-equal-to 4.5b. Modulated structures are absent for specimens with Pb more than 40% (0.4 less-than-or-equal-to x). All of the modulated structures have body-centered orthorhombic symmetries (Mode I) and no transition to base-centered symmetry (Mode II) is observed even by high Pb-substitution in the present (Bi1-xPbx)2Sr2CoOy system, although transition to Mode II is generally observed for superconducting Pb-doped Bi-Sr-Cu-O systems.