Phase identification of the new 126 K Ba-Ca-Cu-O superconductor

New superconducting oxides that have a composition of Ba2Ca2Cu3(CaxCuy)O8+delta with 0.4 greater than or equal to x greater than or equal to 0.9 and 0.7 greater than or equal to y greater than or equal to 0; and show an optimum T-c similar to 126 K were identified. Structural analysis, based on X-ray diffraction, indicated a common tetragonal unit cell among these oxides, which has lattice constants a = 3.85 Angstrom and c = 28.2 Angstrom with an I/4mmm symmetry. The cell consists of a rock-salt (BaO)(2) block sandwiched between two perovskite Ca2Cu3O6 blocks with interstitial Ca-x(+2) and Cu-y(+2) located, e.g. at the 4d site between the two BaO planes. In other words, a so-called 0223 cell with interstitial CaxCu(y). The oxides differ from one another by the amounts of the inserted cations, i.e. the values of x and y. The presence of these interstitial cations offers a new way to adjust the carrier concentration. These oxides are unstable against the incorporation of H2O and CO2 in open air. The decayed products (Ca,Cu,H,CO2)(2)Ba2Ca2Cu3O8+delta have an expanded lattice constant c similar to 33.8 Angstrom and a lower T-c similar to 90 K. A structure model for the decayed phases was also proposed. (C) 1998 Elsevier Science B.V.