X-RAY SINGLE-CRYSTAL STRUCTURE REFINEMENT OF THE 129-K SUPERCONDUCTORHGXPB1-XBA2CA3CU4O10+DELTA

Crystals of Hg0.8Pb0.2Ba2Ca3Cu4O10,14 were measured on an X-ray four-c ircle single-crystal diffractometer and the crystal structure was refi ned to R = 0.023. The space group is P4/mmm and the lattice parameters are a = 3.8530(5) Angstrom, and c = 18.968(3) Angstrom. Lead is incor porated at the Hg site, but it is shifted slightly from the origin in the z = 0 plane. A model of Pb4+ clusters in the basal plane is propos ed, which is in agreement with recent TEM results. A reduction of elec tron density at the Hg site is attributed to the substitution by Cu or CO3 groups. Stacking faults of 1223 or 1245 material are manifested i n the Fourier maps, and it was possible to refine the amount to 3.8%. However, these stacking faults induce additional electron density at t he O(4) site (1/2, 1/2, 0), and cause an overestimation of the excess oxygen content, if it is not corrected carefully. The bondlengths and distances are in agreement with those of other members of the Hg-12 (n -1) n series, and show systematic changes with increasing number n of CuO2 layers. The effect of stacking faults on the diffraction of all m embers of the Hg-12 (n-1) n family is discussed, and a method for thei r determination is given.