Structure, doping and superconductivity in HgBa2CaCu2O6+δ (Tc ≤ 128 K)

We have studied the defect structure and superconducting properties of HgBa2CaCu2O6+δ by neutron powder diffraction, AC susceptibility and DC resistivity. An as-synthesized sample has an onset Tc of 128 K, the highest yet reported for this material. The critical temperature decreases slightly (126 K) after oxygenation, and drops to 92 K after reduction in flowing argon. Neutron diffraction data give evidence that the primary doping mechanism in this material is provided by oxygen atoms in an interstitial position, similar to the case of the one-layer compound HgBa2CaO4+δ. The occupancy of this site varies from 0.08 (1) for the argon-reduced sample to 0.22(1) for the oxygenated sample. Unlike the case of the one-layer compound, no evidence of an additional defect site was found in these samples, HgBa2CaCu2O6+δ has remarkably long copper-apical oxygen distances and almost perfectly flat CuO2 planes