Evidence For The Existence Of 2 Separate 30 Angstrom Phases In The Bi-Sr-Ca-Cu-O System

In the system Bi2O3-SrO-CaO-CuO equilibrium phase-relations were determined in order to elucidate the solid solution range of the 30 angstrom phase. A series of samples was synthesized in the temperature range 800-900-degrees-C. The products were investigated by optical microscopy, scanning electron microscopy, microprobe analysis, X-ray diffractometry and resistance measurements.It was found that the chemistry of the 30 angstrom phase is more complicated than indicated by the popular acronym ''2212'': (1) in the compositional range of the 30 angstrom phase two separate single-phase fields exist: an almost stoichiometric ''B''-phase and a non-stoichiometric ''N''-phase deficient in (Ca+Sr); (2) for both the ''B'' -and the ''N'' -phase the Ca/ (Ca + Sr)-ratios vary within wide limits; (3) there are several striking analogies with the properties of the homologous ''2201'' -phases with c=24 angstrom, as outlined by Roth et al. [J. Mater. Res. 4 (1990) 46].In the range of the 24 angstrom and 30 angstrom compositions the following four phases are identified: 24 angstrom: A-phase ((Sr+Ca) almost-equal-to stoichiometric, monoclinic, not superconducting); 24 angstrom: R-phase (deficient in (Sr+Ca), orthorhombic, superconductor); 30 angstrom: B-phase ((Sr+Ca) almost-equal-to stoichiometric, orthorhombic, not superconducting); 30 angstrom: N-phase (deficient in (Sr+Ca), orthorhombic, superconductor).