Effect of Pb doping on the structure of HgBa₂CuO_4+δ single crystals

X-ray structure analyses were performed with HgBa2CuO4+δ and Hg1-xPbxBa2CuO4+δ single crystals. The onset of the superconducting transition of the crystals ranges from 78-92 K. Hg-1201 crystallizes with space group P4/mmm and has lattice parameters a = 3.8907(2) Å and c = 9.556(1) Å. 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 lattice parameters to a = 3.8836(2) Å and c = 9.553(1) Å, and the refinement converged at R = 0.042 (wR2 = 0.088). Pb atoms are shifted by 0.5 Å to (0.14,0,0) and the excess oxygen atom O(3) by 0.45 Å to (0.38, 1/2, 0). It has not a well defined position in the lattice, but is distributed randomly in a ring-like configuration around the (1/2, 1/2, 0) site, supporting a Pb clustering model. Rather high anisotropic temperature factors U11 and U22 of Hg and apical O(1) atoms are observed, which are also present in higher members of the Hg(Pb)-12(n - 1)n series. They are discussed in terms of corrugated 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 level in 1201.