PHASE-STABILITY LIMITS AND SOLID-STATE DECOMPOSITION OF BI2SR2CACU2O8+DELTA AND BI2SR2CA2CU3O10+DELTA IN REDUCED OXYGEN PRESSURES

We measured the phase stability of the Bi2Sr2CaCu2O8+delta and Bi2Sr2C a2Cu3O10+delta superconductors as a function of temperature and oxygen pressure. Below the solidus temperature of approximately 790-degrees- C, Bi2Sr2CaCu2O8+delta is stable at oxygen pressures one fourth of tho se where CuO becomes metastable, while Bi2Sr2Ca2Cu3O10+delta is stable at the same pressures where CuO is stable. Above the solidus temperat ure, the pressure required to stabilize either phase increases rapidly with temperature. The major solid-state decomposition products of Bi2 Sr2CaCu2O8+delta are Bi2Sr3-xCaxO6+delta and Cu2O, and also Bi2Sr2-xCa xO5 for bismuth-rich samples. For Bi2Sr2Ca2Cu3O10+delta the major deco mposition products are Bi2Sr2CaCu2O8+delta, Ca2-xSrCuO3, and Cu2O. The solid-state decomposition reaction is reversible for the Bi-2212 phas e, but not for the Bi-2223 phase.