High-T_c Superconductivity in Copper Oxide Compounds

The effect of substitution of Sr, Cs, Zn, Ni, P, Cl, S, Se or I for Y, Ba, Cu or 0 on the superconducting transition in the Y-Ba-Cu-0 system has been investigated. It was found that the partial replacement of Cu by Ni or Zn strongly depresses the superconducting transition temperature. An extremely sharp transition was observed in YBa_2Cu_30_y/ samples when they were sintered in iodine gas and then annealed in air or flowing oxygen. The result suggests that the Cu and 0 ions play important roles in the occurrence of the high-T_c superconductivity. An anomalous resistivity change, reminiscent of a superconducting transition, was observed around 180 K in a YBa_2Cu_30_y sample which was sintered in selenium gas followed by annealing in oxygen gas.