INSTABILITY OF Y-EARTH-SUBSITUTED AND RARE-EARTH-SUBSTITUTED BI(PB)-2223 PHASE

Detailed synthetic efforts aimed at incorporating Y/rare-earth ions (R E) into the 110K superconducting phase (Bi,Pb)2Sr2Ca2Cu3O10+delta indi cate that it is not stable in the presence of even small amounts of Y/ RE. The products invariably contain Bi-2212, with Y/RE substituting pa rtially or fully at Ca-sites, as the major phase, but no measurable am ounts of Bi(Pb)-2223 phase. We suggest that this is due to a combinati on of both thermodynamic and kinetic factors; formation rates of 2223 are very slow at the best of times but, in the presence of rare earths , the Bi-2223:RE phase is thermodynamically less stable than an assemb lage containing 2212:RE as the main phase. The complete disappearance of the 2223 phase (viz. Bi1.7Pb0.3Sr2Ca2Cu3O10+delta) upon reaction wi th Bi2Sr2YCu2O8+delta to give non-superconducting Bi2Sr2(Ca,Y)Cu2O8+de lta further supports our conclusion.