Thermal stabilization of impurity HgCaO2 and superconducting phase Hg(Sn)Ba2Ca2Cu3O8+δ in Hg0.9Sn0.1Ba2Ca2Cu3O8+δ superconductor

Tin-doped samples Hg0.9Sn0.1Ba2Ca2Cu3O8+δ ((Hg0.9Sn0.1)-1223) were synthesized in sealed silica tubes by a solid-state reaction method. The dominant phase involved in the sample is a superconducting phase of Hg(Sn)Ba2Ca2Cu3O8+δ (Hg(Sn)-1223), which is a solid solution of Sn in the compound HgBa2Ca2Cu3O8+δ. An impurity phase HgCaO2 also exists together with the dominant phase. The results of thermogravity analysis and XRD analysis show that the impurity phase HgCaO2 and superconducting phase Hg(Sn)-1223 have different decomposition temperatures in flowing He gas and in flowing O2 gas atmosphere. The onset decomposition temperatures of these two phases measured in flowing He gas are 560 and 760 deg. C respectively. They increase to 576 and 825 deg. C respectively in flowing O2 gas. Studies of the effect of annealing temperature on Tc by resistance measurement for this sample show that the superconductivity is reversible in flowing O2 gas below the decomposition temperature of the superconducting phase Hg(Sn)-1223, namely 825 deg. C. The quality of the superconductor, including the critical temperature and the sharpness of the transition, can be improved by removing the impurity phase HgCaO2 from the sample using a high-temperature annealing process under flowing O2 gas. (author)