Oxygen distribution, incommensurate modulation, and structural disorder in Bi2Sr2Ca1-yCu2O8+delta and Bi10Sr15Fe10O46 single crystals

Electron diffraction and high-resolution electron microscopy along the c axis have been used to examine the local structural variations in modulated Bi(2)Sr(2)Ca(1-y)Y(y)Cu(2)O8+delta and Bi10Sr15Fe10O46 single crystals in the as-prepared, underdoped, and overdoped state. In both materials the structural disorder, of which the structural modulation is one aspect, results from the labile nature of the BiO planes. A structural model for the BiO layers which explains the structural modulation and describes all the structural defects observed is suggested. A drastic change in the nature of the modulation due to an antiphase boundary structure appears in the highly substituted Y crystals in concomitance with the metal-insulator transition. The modified modulation contributes to the suppression of superconductivity by affecting the geometry of the CuO planes.