We have found superconductivity in the 90-K range in ^Ba2Cu306+x with A = La, Nd, Sm, Eu, Gd, Ho, Er, and Lu in addition to Y. The results suggest that the unique square-planar Cu atoms, each surrounded by four or six oxygen atoms, are crucial to the superconductivity of oxides in general. In particular, the high T c of /!Ba2Cu3C>6+x is attributed mainly to the quasi two-dimensional assembly of the Cu02-Ba-CuC>2+x-Ba-Cu02 layers sandwiched between two A layers, with particular emphasis in the Cu02+* layers. Higher-T c oxides are predicted for compounds with bigger assemblies of Cu02 layers coupled by Ba layers.

Four phases are observed in superconducting Bi-Ca-Sr-Cu-0 samples. The superconducting phase, with onset temperature near 120 K, is a 15.4-A-layered compound with composition near Bi&Ca&Sr2Cu209 and an A-centered orthorhombic unit subcell 5.41&5.44&&30.78 A. X-ray diffraction and electron microscopy data are consistent with a structure of alternating perovskite and Bi202 layers. High-resolution transmission electron microscopy images reveal a b-axis superstructure of 27.2 A, numerous (001) stacking faults, and other defects.