Superconductivity of (Hg_0.7Tl_0.3)₂Ba₂Y _1-xCa_xCu₂O_8-δ (0 ≦ x ≦ 0.8) with a wide range of doping state

(Hg,Tl)-based cuprates with the nominal compositions of (Hg_0.7Tl_0.3)₂Ba₂Y _1-xCa_xCu₂O_8-δ (0 ≦ x ≦ 1.0, every 0.1 step) were prepared under high pressure, and nearly single phases were obtained for the samples with 0 ≦ x ≦ 0.8. We systematically investigated their unit cell dimensions, DC susceptibility, electrical resistivity, oxygen content and copper valence as a function of Ca content, x. For the samples with x = 0-0.4, T_c, which was determined by DC susceptibility, increased from 12 K to 84 K with increasing x, while the samples with x = 0.5-0.8 exhibited a T_c decrease from 84 K to 40 K. This result indicates that the doping state of the samples can be extensively varied from underdoped to overdoped states by the Ca substitution. This variation in the doping states was also supported by the systematic change in the temperature dependence of the electrical resistivity. Oxygen analysis revealed that the copper valence gradually increased from 2.12 to 2.26 with increasing x from 0 to 0.8. The copper valence with the maximum T_c of 84 K in these samples was determined to be 2.18-2.19, which is comparable to the value for the optimum T_c in other high-T_c cuprates.