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 fou r-circle single-crystal diffractometer and the crystal structure was r efined to R = 0.033. The onset of the superconducting transition of th e crystals ranges from 101 K-115 K. The space group is P4/mmm and latt ice 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 hea vy atoms are shifted away from the origin by similar to 0.38 ti. Stack ing 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 ad ditional electron density at the ideal O(4) site and mask a possible o xygen occupancy at (1/2, 1/2, 0). However, electron density is observe d 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 dis tances 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.