NEUTRON POWDER DIFFRACTION STUDY AT ROOM-TEMPERATURE AND AT 10-K OF THE CRYSTAL-STRUCTURE OF THE 133-K SUPERCONDUCTOR HGBA2CA2CU3O8+DELTA

The crystal structure of the superconductor HgBa2Ca2Cu3O8+delta has be en determined by neutron powder diffraction techniques at room tempera ture and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.8501 (1), c=15.7837 (9) Angst rom. The structure is described by the sequence of layers: (CuO2)(Ca)( CuO2)(BaO)(HgOdelta)(BaO)(CuO2)(Ca)]... The compound, prepared by a so lid-state reaction between HgO and the precursor Ba2Ca2Cu3Ox at a pres sure of 18 kbar and 880 degrees C, has an oxygen composition correspon ding to delta=0.41 (2). This extra oxygen is located on the layer of t he mercury atoms at the sites 1/2, 1/2, 1/2. The superconducting trans ition temperature for the as-prepared sample is T-c=133 K. No evidence has been found of orthorhombic distortions, nor for substitution of s ome of the Hg atoms by Cu. The copper and oxygen atoms of the layers ( CuO2) are coplanar for the Z=0 (CuO2) plane and almost perfectly copla nar for the others. No phase transition occurs down to 10 K.