X-ray single crystal structure analysis of the 1245 type superconductor Hg_0.5Pb_0.5Ba₂Ca₄Cu₅O_12+δ

Abstract

Crystals of Hg0.5Pb0.5Ba2Ca4Cu5O12+δ 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) Å, and c = 22.172(2) A ̊. Hg is partially substituted for Pb and heavy atoms are shifted away from the origin by ∼0.38 A ̊. 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 Å 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 H-12(n-1)n series. © 1995.