STRUCTURE AND PROPERTIES OF (BI, PB)2SR2(CA, Y)CU2O8+DELTA

On a number of compositions in the solid solutions Bi2-xPbxSr2CuO6+δ and Bi2-xPbxSr2Ca1-yY yCu2O8+δ high resolution electron microscopy, X-ray diffraction, magnetic measurements and oxygen content determination were performed. In general substitution of Ca by Y leads to a decrease in Tc, which can be partly suppressed by the substitution of Bi by Pb. With increasing amount of Pb the modulated structure changes from a Bi-type, via a mixed Bi/Pb-type, and a Pb-type of modulation, to a structure in which the modulation is absent. In the Pb-type the modulation waves in neighbouring (Bi, Pb)O layers are in phase with approximately k = 8b. Over a considerable composition region the Bi-type and Pb-type of modulation coexist in the same region. Models for the Pb-type and the mixed Bi/Pb-type of modulation are postulated, based on the incorporation of extra oxygen in the (Bi, Pb)O layers. It is shown that the balance between the Pb-type and Bi-type of modulation is very delicate, and probably depends on the distribution of the excess oxygen in the two (Bi, Pb)O layers. © 1990.