Superconductivity in the high-Tc Bi-Ca-Sr-Cu-O system: Phase identification.

Four phases are observed in superconducting Bi-Ca-Sr-Cu-O samples. The superconducting phase, with onset temperature near 120 K, is a 15.4-A\r{}-layered compound with composition near ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{1}$${\mathrm{Sr}}_{2}$${\mathrm{O}}_{9}$ and an A-centered orthorhombic unit subcell 5.41\ifmmode\times\else\texttimes\fi{}5.44\ifmmode\times\else\texttimes\fi{}30.78 A${\r{}}^{3}$. X-ray diffraction and electron microscopy data are consistent with a structure of alternating perovskite and ${\mathrm{Bi}}_{2}$${\mathrm{O}}_{2}$ layers. High-resolution transmission electron microscopy images reveal a b-axis superstructure of 27.2 A\r{}, numerous (001) stacking faults, and other defects.