Double (Hg,Tl)-O layer superconductors with three or four CuO2 sheets, (Hg1-xTlx)2Ba2Ca n-1CunOy (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. Tc 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, Tc 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.
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