NEUTRON POWDER DIFFRACTION STUDY OF THE CRYSTAL-STRUCTURE OF HGBA2CA4CU5O12+DELTA AT ROOM-TEMPERATURE AND AT 10-K

The crystal structure of the superconductor HgBa2Ca4Cu5O12+6 has been analyzed by neutron powder diffraction at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.85145(8) and c=22.1066(9) angstrom. The unit cell contains three (CuO2) layers with copper in four-fold planar coordination and two (CuO2) layers with copper in five-fold pyramidal coordination. The Cu-O apical distance for this last copper is 2.85(1) angstrom, i.e. of the same order as those found in other members of the homologous series. The compound has an oxygen composition corresponding to delta=0.32(3). This extra oxygen is located on the layer of the mercury atoms at the sites (1/2, 1/2 0). The superconducting transition temperature for the ''as-prepared'' sample with this composition is 101 K. In the structure of Hg-1245 there is no evidence for substitution of mercury by copper and for the presence of oxygen at the sites (1/2, 0, z). The copper and oxygen atoms are coplanar for the (CuO2) layer at z = 1/2, and almost coplanar for the other layers. No phase transition has been observed down to 10 K. The AC susceptibility versus T curve of the sample used in the experiment shows the presence of a superconducting minority phase with a T(c) almost-equal-to 114 K. This phase could be Hg-1256, whose presence in the sample has been established by neutron and electron diffraction.