Professor Paolo G. Radaelli OSI

Abstract:

We have studied the defect structure and superconducting propertiess 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 HgBa2CuO4+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. © 1993.

Abstract:

We have used neutron powder diffraction to investigate the defect structure of HgBa2CuO4+δ. An interstitial oxygen defect in the Hg plane is the primary doping mechanism. A superconducting transition temperature, Tc onset, of 95 K is achieved when ≈0.06 oxygen atoms per formula unit are incorporated at this site by annealing the sample at 500°C in pure oxygen. Annealing in argon at 500°C lowers the oxygen content in this site to ≈0.01 and results in a Tc of 59 K. The neutron powder diffraction data give evidence for a second defect in the Hg plane which we conclude involves the substitution of copper for about 8% of the mercury and the incorporation of additional oxygen (≈0.1 atoms per formula unit), presumably bonded to the copper defects. In the present samples, the concentration of this defect does not vary with synthesis conditions and its contribution to doping is, therefore, unclear. The structure of the compound is the same at room temperature and superconducting temperatures. © 1993.