THE DEPENDENCE OF BI9SR11CA5 OXIDE ON THE STOICHIOMETRY OF BI-2212 AND ITS REVERSIBLE PHASE-TRANSITION IN OXIDIZING AND NONOXIDISING ANNEALING CONDITIONS

A molar ratio of 2:2:1:2 for Bi, Sr, Ca and Cu, respectively, showed impurity phases after the completion of the synthesis process described here. The sample was decomposed by high-temperature annealing under nitrogen and then the Bi-2212 phase was reformed by reoxygenation but the original impurities remained. The X-ray diffraction results show the predominant impurity phase and the phase after decomposition is a Cu-free compound. The composition is identified as Bi9Sr11Ca5 oxide which goes through a phase transition as a result of uptake and loss. The phase equilibria between the Cu-free composition and Bi-2212 shows the latter can be reduced by lowering the molar ratio of Sr and Ca followed by post annealing in N2 and then O2 and can be removed entirely by using a Bi-rich composition.