EFFECT OF PB DOPING ON THE STRUCTURE OF HGBA2CUO4-CRYSTALS(DELTA SINGLE)

X-ray structure analyses were performed with HgBa2CuO4+delta and Hg1-x PbxBa2CuO4+delta single crystals. The onset of the superconducting tra nsition of the crystals ranges from 78-92 K. Hg-1201 crystallizes with space group P4/mmm and has lattice parameters a = 3.8907(2) Angstrom and c = 9.556(1) Angstrom. The structure was refined to R = 0.023 (wR2 = 0.036) in the ideal 1201 type with an excess oxygen content of 6(2) % and an occupancy of 97% at the Hg site. Pb-doping decreases the latt ice parameters to a = 3.8836(2) Angstrom and c = 9.553(1) Angstrom, an d the refinement converged at R = 0.042 (wR2 = 0.088). Pb atoms are sh ifted by 0.5 Angstrom to (0.14, 0,0) and the excess oxygen atom O(3) b y 0.45 Angstrom to (0.38, 1/2, 0). It has not a well defined position in the lattice, but is distributed randomly in a ring-like configurati on around the (1/2, 1/2, 0) site, supporting a Pb clustering model. Ra ther high anisotropic temperature factors U-11 and U-22 of Hg and apic al O(1) atoms are observed, which are also present in higher members o f the Hg(Pb)-12(n - 1)n series. They are discussed in terms of corruga ted O-Hg-O bonds. Comparison with other structure refinements of 1201 shows a correlation of the transition temperature with the position of the Ba atom, which seems to be a good indicator for the hole doping l evel in 1201.