Effects of metal substitution on superconductivity and crystal structure of HgBa₂CuO_4+δ

High-T_c superconducting oxides with nominal compositions of HgBa₂Cu_1-xM_xO_4+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 T_c 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 T_c 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 La_2-xSr_xCuO₄ and YBa₂Cu₃O_6+δ.