Simple and double modulations in Bi2-xPbxSr2CaCu2O8+delta - art. no. 214511

Two single crystals of the Pb-doped high-T-c superconductor Bi2-xPbxSr2CaCu 2O8 + delta 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 characteri zed by a double modulation (9(I)=0.2b*+c*; q(II)=0.15b*). The two modulated structures have been refined from single crystal x-ray diffraction data us ing the four- and five-dimensional superspace groups' formalism, respective ly. Considering these substituted phases and the well-known lead-free phase , the same symmetry is observed for their average structures, while their s tructural modulations exhibit significant discrepancies. Different disorder ed regions are characterized in relation to the modulation period. The resu lts are confirmed by high-resolution electron-microscopy observations. The relative structures are compared with each other and the structure of the l ead-free phase. particularly about the stacking scheme of the structural la yers.