Incommensurate Modulations in the Pb-Doped BiSrCaCuO 221 Superconducting Phase: A Five-Dimensional Superspace Description

The new modulation introduced by Pb doping of the 221 BiSrCaCuO superconducting phase has been analyzed by using five-dimensional superspace group theory. The complete X-ray diffraction pattern has monoclinic rather than orthorhombic symmetry and is in agreement with the superspace group P:Aa:P1 In addition to the original modulation with q₁ = 0.234 (1)a*, an extra set of satellites with q₂ = 0.144 (3)a*, and “combination” satellites with q₁ – q₂ = 0.090 (1)a* occur. The two modulations differ in the internal translational symmetry. Anomalous dispersion experiments show that both Bi and Pb atoms are affected by each of the modulations, a conclusion confirmed by the diffraction analysis. The displacements of the new modulation are in-phase, rather than out-of-phase, for two adjacent BiO layers. The c-axis displacements of the Bi atoms for the original modulations are significantly reduced, while the amplitudes of the q₂ modulation in this direction exceed 0.3 Å for several of the atoms.