X-RAY SINGLE-CRYSTAL STRUCTURE REFINEMENT OF THE 129-K SUPERCONDUCTOR HGXPB1-XBA2CA3CU4O10+DELTA

Crystals of Hg0.8Pb0.2Ba2Ca3Cu4O10,14 were measured on an X-ray four-circle single-crystal diffractometer and the crystal structure was refined 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 incorporated 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 proposed, which is in agreement with recent TEM results. A reduction of electron density at the Hg site is attributed to the substitution by Cu or CO3 groups. Stacking faults of 1223 or 1245 material are manifested in the Fourier maps, and it was possible to refine the amount to 3.8%. However, these stacking faults induce additional electron density at the 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 members of the Hg-12 (n-1) n family is discussed, and a method for their determination is given.