Superconductivity at 80-90 K has been resistively and magnetically observed in the $R$-Tl-Sr-Ca-Cu-O systems ($R$ represents the rare earths including Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and excepting Ce) in which five metal elements all are required for the superconductivity. In particular, the Pr-Tl-Sr-Ca-Cu-O system and Tb-Tl-Sr-Ca-Cu-O system are the first superconducting systems above 77 K in which Pr or Tb are required for the superconductivity. We believe that even higher critical temperatures may be achieved by optimizing the starting composition and preparation conditions, and/or by further elemental substitutions.

• Received 19 September 1988

DOI:https://doi.org/10.1103/PhysRevB.39.2918