OPTIMIZATION OF SUPERCONDUCTING CRITICAL-TEMPERATURES BY CONTROL OF CATION AND ANION STOICHIOMETRY IN BI2SR2CACU2O-DELTA-BASED SOLID-SOLUTIONS

T-c data are reported for powders of cation-stoichiometric Bi2Sr2CaCu2Odelta and for nonstoichiometric samples based on the three mechanisms Bi reversible arrow Sr, Sr reversible arrow Ca a nd Sr vacancy. For each, the T-c values depend critically on the final oxygen contents, which were varied by heating samples in either O-2 or N-2 at different temperatures. Stoichiometric Bi2Sr2CaCu2Odelta has the highest T-c, 92 K, obtained after heating in O-2 at similar to 820 degrees C. Heating in O-2 at lower temperatures gives rise to overdoping and T-c decreases to 60 K. The other cation compositions show a smaller maximum T-c but also less reduction in T-c on overdoping. Under-doped samples, with reduced T-c values were obtained on heating in N-2. These data, together with selected literature results, lead to a unified picture of the variation of T-c with cation composition and oxygen content.