STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF (HG1-XTLX)(2)BA2CAN-1CUNOY (N=3, 4) SUPERCONDUCTORS PREPARED UNDER HIGH-PRESSURE

Double (Hg;Tl)-O layer superconductors with three or four CuO2 sheets, (Hg1-xTlx)(2)Ba2Can-1CunOy (x = 0.3-0.6; n = 3, 4), have been synthes ized under high pressure and high temperature. These compounds are mem bers of a new homologous series, (Hg,Tl)-22(n-1)n. The effects of Tl c ontent 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 phase s increases from 45 K to 81 K with an increase in x, and that of Ar-an nealed samples increases up to similar to 90 K, accompanied with an ex pansion 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 (similar to 115 K) irrespective ly of both the Tl content and Ar-annealing, suggesting that they are n early optimally doped.