We report the discovery of three new phases in the Tl-Ca-Ba-Cu-O system, ${\mathrm{Tl}}_1{\mathrm{Ca}}_{n-1\mathrm{}}{\mathrm{Ba}}_2{\mathrm{Cu}}_n{\mathrm{O}}_{2n+3}$ $n=1,2,3\mathrm{}$, one of which $n=3\mathrm{}$ exhibits bulk superconductivity at ≃100 K. The crystal structures of these new materials are closely related to that of ${\mathrm{Tl}}_2{\mathrm{Ca}}_2{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$, the recently discovered 125-K superconductor. All of these compounds contain Cu perovskitelike units, but whereas these are separated by bilayer Tl-O sheets in the 125-K material, they are separated by monolayer Tl-O sheets in the new structures, enabling an examination of the effect of the coupling between successive Cu perovksite units on superconducting properties.

• Received 31 March 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.750