A model for the modulation in Bi2−xPbxSr2CaCu2O8+δ with large x

Abstract

Compounds with composition Bi2−xPbxSr2CaCu2O8+∂ show to have two different kinds of incommensurate superstructures. The first one (Bi-type) occurs for x being small and for the pure Bi compound. This superstructure is reported by many authors, which all reports shifts of all atoms. The modulation is directed along the b axis and the modulation vector was found to vary between about 3.5b and 5.0b for Ca completely replaced by Y and Bi2−xPbxSr2CaCu2O8+∂ with x being 0.1 respectively. A model for the modulation was proposed by Zandbergen et al, based on the interpretation of HREM images and structural chemistry arguments. This model, assuming the introduction of extra oxygen in the BiO layers to decrease the mismatch between the BiO layers and the perovskite blocks with which the BiO layers are sandwiched, has been proven to be basically right and is schematically represented in Figure 1.For x being larger than about 0.2 the amplitude of the Bi type of modulation starts to decrease and another type of modulation (Pb-type) is occurring. A number of authors have reported this type of of modulation but up till now no explanation has been given. The modulation vector is mostly reported to be close to 7.5b but also a variation with x is reported. When x is between 0.2 and 0.5 both types of modulation are present and Pierre et al report that when both modulation are present the ratio of both modulation vectors is 3/2. Similarly to the Bi type of modulation, the Pb type of modulation is accompanied with a periodicity of 0.54 nm along the a axis, as is shown in Figure 2.