High-pressure synthesis of a new homologous series of superconductors, (Hg_1-xTl_x)₂Ba₂Ca _n-1Cu_nO_y (0.3 ≤ x ≤ 0.6, n=3,4)

We have recently synthesized new superconducting cuprates containing a mercury-based bilayer, i.e. (Hg, Tl)₂Ba₂(Ca, Y)Cu₂O_y and (Hg_1-xTl_x)₂Ba₂Ca _n-1Cu_nO_y (x ∼ 0.4, n = 3, 4) by a high-pressure technique. These compounds are the n=2, 3 and 4 members of a new homologous series, (Hg, Tl)-22(n - 1)n. The effects of Tl substitution for Hg on the formation and superconductivity of the (Hg, Tl)-2223 and -2234 phases are investigated. Both the phases are stabilized for the Tl content x of 0.3-0.6. T_c of the as-synthesized (Hg, Tl)-2223 compound increases from 45 K to 81 K with an increase in x. In contrast, T_c of the (Hg,Tl)-2234 compound is almost independent of x (∼114 K).