SYNTHESIS AND NEUTRON POWDER DIFFRACTION STUDY OF THE SUPERCONDUCTOR HGBA2CACU2O6+DELTA BEFORE AND AFTER HEAT-TREATMENT

The crystal structure of the superconductor HgBa2CaCu2O6+delta has been analyzed by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a = 3.8526(2), c = 12.6367(8) angstrom. The structure is made of the sequence of layers ...[(BaO)(CuO2)(Ca)(CuO2)(BaO)(HgO(delta))]... The compound, prepared by a solid-state reaction between HgO and the precursor Ba2CaCu2Ox at 18 kbar and 880-degrees-C, has an oxygen content corresponding to delta = 0.35(2) and a transition temperature of T(c) congruent-to 104 K. Annealing in O2 (300-degrees-C for 20 h) reduces the oxygen content to delta = 0.28(2). A sample annealed under the same conditions or in N2 (250-degrees-C for 12 h) was found to have a T(c) of 123 K. Additional annealing in N2 at 400-degrees-C for 18 h lowers T(c) to 110 K. Our results indicate that the material with delta = 0.35 is overdoped. The oxygen above the O6 stoichiometry is located on the Hg layers of the structure, at the positions 1/2, 1/2, 0. No evidence has been found of the substitution of mercury by copper. The copper and oxygen atoms of the (CuO2) layers are almost exactly coplanar in this compound.