X-RAY SINGLE-CRYSTAL STRUCTURE-ANALYSIS OF THE 1245-TYPE-SUPERCONDUCTOR HG0.5PB0.5BA2CA4CU5O12+DELTA

Crystals of Hg0.5Pb0.5Ba2Ca4Cu5O12+delta were measured on an X-ray four-circle single-crystal diffractometer and the crystal structure was refined to R = 0.033. The onset of the superconducting transition of the crystals ranges from 101 K-115 K. The space group is P4/mmm and lattice parameters of the five-layer material are a = 3.8529(3) Angstrom, and c = 22.172(2) Angstrom. Hg is partially substituted for Pb and heavy atoms are shifted away from the origin by similar to 0.38 ti. Stacking faults of 1234 or 1256 material are manifest in the Fourier maps, and the amount was refined to 4.4%. These stacking faults induce an additional electron density at the ideal O(4) site and mask a possible oxygen occupancy at (1/2, 1/2, 0). However, electron density is observed at a distance of 0.6 Angstrom from(1/2, 1/2, 0), suggesting a shift of the excess oxygen atom from its ideal position. Bondlengths and distances show systematic changes with increasing number n of CuO2 layers. Changes occur mainly in the rock-salt type layer, whereas the CaCuO2 units remain stable over the whole Hg-12(n - 1)n series.