Crystal structure of annealed and as-prepared HgBa₂CaCu₂O_6+δ superconductors

Neutron profile refinements have been carried out on powders of as-prepared and oxygen annealed samples of the layered HgBa₂CaCu₂O_6+δ (1212) cuprate superconductors. The crystal structure is tetragonal P4/mmm over the range of temperatures 10 K to 298 K investigated, with no structural transitions of any kind being detected. The extra oxygen that electrically dopes these materials is found to reside in the centered position of the Hg plane, and no other excess oxygen is present in these samples. In addition, we found no conclusive evidence for mixing of the Hg and Cu cations. In the as-prepared sample we found δ=0.22, while the oxygen content increased substantially to δ=0.35 with annealing. This extra oxygen had only a small effect on the superconductivity, broadening the transition somewhat and shifting T_c from 112 K to 120 K. The incorporation of extra oxygen also had the effect of constructing the lattice parameters by ∼ 0.1%. The overall behavior is similar to the single-layer Hg-1201 system.