Bulk superconductivity at 125 K in Tl2Ca2Ba2Cu3Ox

Bulk superconductivity is reported in Tl2Ca2Ba2 Cu3Ox with a transition to zero resistance at 125 K, the highest transition temperature (Tc) 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. Tc 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 Tc. A second structure containing only bilayer perovskitelike units Tl2Ca1Ba2Cu2Ox is a bulk superconductor with Tc ranging from 95 to 108 K. © 1988 The American Physical Society.