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

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 an d 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 coor dination and two (CuO2) layers with copper in five-fold pyramidal coor dination. The Cu-O apical distance for this last copper is 2.85(1) ang strom, i.e. of the same order as those found in other members of the h omologous series. The compound has an oxygen composition corresponding to delta=0.32(3). This extra oxygen is located on the layer of the me rcury atoms at the sites (1/2, 1/2 0). The superconducting transition temperature for the ''as-prepared'' sample with this composition is 10 1 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 transi tion has been observed down to 10 K. The AC susceptibility versus T cu rve of the sample used in the experiment shows the presence of a super conducting minority phase with a T(c) almost-equal-to 114 K. This phas e could be Hg-1256, whose presence in the sample has been established by neutron and electron diffraction.