Space symmetry of the basic structure and incommensurate modulations in (Pb,Bi)/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ superconductor single crystals

Lead-containing single crystals of the (2:2:1:2) phase have been studied on the basis of monochromatic x-ray precession camera investigations. Compared to the lead-free compound it is shown that the lead addition only affects the incommensurate properties preserving the space symmetry of the basic structure. The presence of two incommensurate modulations is pointed out, consistent with recent results, and their characteristics are specified. The type of modulation which also exists in the lead-free compound has its wavelength and its anharmonicity reduced by the lead addition. The other modulation is specific of lead-containing compounds and has the wave vector (0,(1/6.4,0)). These results suggest that the lead atoms are substituted in part by the Bi atoms in the Bi-O layers of the structure.