Structural and superconducting properties of (Hg_{1 - X}Tl_x)₂Ba₂Ca_n-1Cu _nO_y (n = 3, 4) superconductors prepared under high pressure

Double (Hg,Tl)-O layer superconductors with three or four CuO₂ sheets, (Hg_1-xTl_x)₂Ba₂Ca _n-1Cu_nO_y (x = 0.3-0.6; n = 3, 4), have been synthesized under high pressure and high temperature. These compounds are members of a new homologous series, (Hg,Tl)-22(n - 1)n. The effects of Tl content on the formation and superconductivity of the (Hg,Tl)-2223 and -2234 phases are investigated. Both the phases are stabilized for the nominal Tl content x of 0.3-0.6. T_c of as-sintered (Hg,Tl)-2223 phases increases from 45 K to 81 K with an increase in x, and that of Ar-annealed samples increases up to ∼ 90 K, accompanied with an expansion of the c-axis. These changes can be explained considering that as-synthesized 2223 phases are highly overdoped. In contrast, T_c of (Hg,Tl)-2234 phases is almost unchanged (∼ 115 K) irrespectively of both the Tl content and Ar-annealing, suggesting that they are nearly optimally doped.