Synthesis and neutron powder diffraction study of the superconductor HgBa2CaCu2O6+δ before and after heat treatment

Abstract The crystal structure of the superconductor HgBa2CaCu2O6+δ 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) A. The structure is made of the sequence of layers …[(BaO)(CuO2)(Ca)(CuO2)(BaO)(HgOδ)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba2CaCu2Ox at 18 kbar and 880°C, has an oxygen content corresponding to δ=0.35(2) and a transition temperature of T c ⋍ 104 K . Annealing in O2 (300°C for 20 h) reduces the oxygen content to δ=0.28(2). A sample annealed under the same conditions or in N2 (250°C for 12 h) was found to have a Tc of 123 K. Additional annealing in N2 at 400°C for 18 h lowers Tc to 110 K. Our results indicate that the material with δ=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.