Bulk Superconductivity at 125 K in ${\mathrm{Tl}}_2{\mathrm{Ca}}_2{\mathrm{Ba}}_2{\mathrm{Cu}}_3{O}_x$

Bulk superconductivity is reported in ${\mathrm{Tl}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ with a transition to zero resistance at ≃ 125 K, the highest transition temperature ($T_c$) yet found. Transmission electron spectroscopy shows that the unit cell is body-centered tetragonal and contains trilayer Cu perovskitelike units separated by bilayer Tl-O units. $T_c$ in this material varies from 118 to 125 K depending on preparation conditions. A high density of perovskitelike bilayer intergrowths are observed in the 118-K material, which we speculate result in the decreased $T_c$. A second structure containing only bilayer perovskitelike units—${\mathrm{Tl}}_2{\mathrm{Ca}}_1{\mathrm{Ba}}_2{\mathrm{Cu}}_2{O}_x$—is a bulk superconductor with $T_c$ ranging from 95 to 108 K.