Instability of Y- and rare-earth-substituted Bi(Pb)-2223 phase

Abstract

Detailed synthetic efforts aimed at incorporating Y/rare-earth ions (RE) into the 110 K superconducting phase (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +δ} indicate 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 partially or fully at Ca-sites, as the major phase, but no measurable amounts of Bi(Pb)-2223 phase. We suggest that this is due to a combination 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 assemblage containing 2212:RE as the main phase. The complete disappearance of the 2223 phase (viz. Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_{10 +δ}) upon reaction with Bi₂Sr₂YCu₂O_{8 +δ} to give non-superconducting Bi₂Sr₂(Ca,Y)Cu₂O_{8 +δ} further supports our conclusion.