Phase equilibria in the quinary BSCCO

Abstract

Melting relations of Bi-2212 and Bi-2201 as a function of oxygen pressure are explicable in terms of increasing amounts of univalent copper in the liquids under more reducing conditions. The effects of the change of copper valence are manifold and not easily predicted. The melting temperatures of cuprates in the Bi-2212 composition results in rotation of the phase relations so Bi-2212 melts along five different liquid lines of descent. Furthermore, Bi-2212 melts at different isobaric invariant points and, in one narrow range of oxygen pressure, on an isobaric univariant line. In this pressure range the amount of liquid produced at the solidus is maximized and only two crystalline phases exist in the supersolidus. As these are both alkaline earth cuprates, all Bi is in the liquid in this region. The maximum Cu content of Bi-2201 is strongly influenced by temperature - only in a narrow temperature range near the solidus does stoichiometric Bi-2201 exist. Lowering oxygen pressure results in a depression of the melting temperature so that if the partial pressure of oxygen is less than 0.5 bars, stoichiometric Bi-2201 does not form. In addition to the limitation of this phase to Cu-deficient compositions, the lowering of oxygen pressure causes a decrease in size of the CuO primary phase field. The Bi-2201-CuO join is a thermal divide at higher oxygen pressures but is pierced at lower values.