NEUTRON POWDER DIFFRACTION STUDY OF THE CRYSTAL-STRUCTURES OF HGBA2CUO4+DELTA AND HGBAO2

The crystal structures of HgBa2CuO4+delta and HgBaO2 have been refined at room temperature using neutron powder diffraction data obtained from a sample containing 44% of the first phase, 42% of the second and 14% of Ba2Cu3O5+delta. the compound HgBa2CuO4+delta crystallizes with the symmetry of space group P4/mmm and lattice parameters a = 3.8829 (6), c = 9.5129 (14) angstrom. The unit cell contains only one CuO2 layer and the material is a superconductor with a value of T(c) of 94 K. The oxygen in excess of the O4 stoichiometry (delta = 0.063 in our sample) is located in interstitial positions on the HgO(delta) layer, and it is the only extra oxygen present in the structure. No mixing of the cations has been detected in this study. The compound HgBaO2 crystallizes with the symmetry of space group R3mBAR and lattice parameters (hexagonal axes) a = 4.0991 (6), c = 19.355 (3) angstrom. In this new polymorph of HgBaO2 the Ba atoms are octahedrally coordinated while Hg has two-fold coordination.