Simple and double modulations in Bi2- xPbxSr2CaCu2O8+δ

Two single crystals of the Pb-doped high-T_c superconductor Bi_{2- x}Pb_xSr₂CaCu₂O_8+δ phase have been isolated with x=0.4 and 0.2. In the first case, the structure is monoincommensurately modulated with the so-called Pb-type modulation (q_II=0.11b^*). In the second case, the structure is characterized by a double modulation (q_I=0.2b^*+c^* q_II=0.15b^*). The two modulated structures have been refined from single-crystal x-ray diffraction data using the four- and five-dimensional superspace groups' formalism, respectively. Considering these substituted phases and the well-known lead-free phase, the same symmetry is observed for their average structures, while their structural modulations exhibit significant discrepancies. Different disordered regions are characterized in relation to the modulation period. The results are confirmed by high-resolution electron-microscopy observations. The relative structures are compared with each other and the structure of the lead-free phase, particularly about the stacking scheme of the structural layers.