High-Tc superconducting oxides with nominal compositions of HgBa2Cu1-xMxO4+δ (M=Fe, Co, Ni, Zn and Mg, x=0-0.03) were prepared, and crystal structure and substitution effects on superconducting properties were studied using X-ray powder diffraction, susceptibility and resistivity. Almost all single-phase samples were obtained for M=Fe, Co, Zn and Mg, while impurities such as Ni3Ba3O8 were detected for M=Ni. A significant decrease in Tc and an increase in lattice parameter a for the samples with Zn and Mg indicated that these metals were successfully substituted for Cu. On the other hand, there was no remarkable change in Tc for the sample with Fe and Co, which suggests a possibility of substitution for Hg. Zn- and Mg-substitution effects on superconductivity were discussed from the viewpoint of structural change, in comparison with the cases of La2-xSrxCuO4 and YBa2Cu3O6+δ.
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