Solid‐Solution Ranges of the <i>n</i> = 2 and <i>n</i> = 3 Superconducting Phases in Bi ₂ (Sr _<i>x</i> Ca _{1 − <i>x</i>} ) _{<i>n</i> +1} Cu _<i>n</i> O _<i>y</i> and the Effect on <i>T</i> _c

Abstract

The cation solubility limits of the n = 2 and n = 3 superconducting phases in the Bi 2 (Sr x Ca 1 − x ) n +1 Cu n O y system were established along tie lines with compatible phases via electron probe microanalysis on bi‐ (or poly‐) phasic samples prepared at 860°C. Pb additions (15 mol% of the Bi content) were used to facilitate formation of the n = 3 phase. In each case football‐shaped volumes in composition space were established as the solubility limits which bordered on the nominal compositions 2212 or 2223 (Bi:Sr:Ca:Cu) with the long axis parallel to the Sr‐Ca side of the quaternary (i.e., Sr‐to‐Ca intersolubility) but also extending toward Bi and Cu. This means that, for the most part, the superconducting phases are alkaline‐earth deficient relative to the ideal 2212 and 2223 compositions. The Pb content in the 2223 phase is typically 10% of the Bi content. T c variations could be correlated with variations in Sr or (Sr + Bi) content and the length of the c ‐axis parameter.