Evidence for the existence of two separate 30 Å phases in the BiSrCaCuO system

Abstract

In the system Bi 2O 3SrOCaOCuO equilibrium phase-relations were determined in order to elucidate the solid solution range of the 30 Å phase. A series of samples was synthesized in the temperature range 800–900°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 Å phase is more complicated than indicated by the popular acronym “2212”: (1) in the compositional range of the 30 Å phase two separate single-phase fields exist: an almost stoichiometric “B”-phase and a nonstoichiometric “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 A ̊ , as outlined by Roth et al. [J. Mater. Res. 4 (1990) 46]. In the range of the 24 Å and 30 Å compositions the following four phases are identified: 24Å: A-phase ((Sr+Ca)≈stoichiometric, monoclinic, not superconducting); 24 Å: R-phase (deficient in (Sr+Ca), orthorhombic, superconductor); 30 Å: B-phase ((Sr+Ca)≈stoichiometric, orthorhombic, not superconducting); 30 Å: N-phase (deficient in (Sr+Ca), orthorhombic, superconductor).