Effect of chemical inhomogeneity in bismuth-based copper oxide superconductors

We examine the effect on the superconducting transition temperature ${(T}_{\mathrm{c}})$ of chemical inhomogeneities in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CuO}}_{6+\delta }$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ single crystals. Cation disorder at the Sr crystallographic site is inherent in these materials and strongly affects the value of $T_{\mathrm{c}}.$ Partial substitution of Sr by Ln (Ln = La, Pr, Nd, Sm, Eu, Gd, and Bi) in ${\mathrm{Bi}}_2{\mathrm{Sr}}_{1.6}{\mathrm{Ln}}_{0.4}{\mathrm{CuO}}_{6+\delta }$ results in a monotonic decrease of $T_{\mathrm{c}}$ with increasing ionic radius mismatch. By minimizing Sr site disorder at the expense of Ca site disorder, we demonstrate that the $T_{\mathrm{c}}$ of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ can be increased to 96 K. Based on these results we discuss the effects of chemical inhomogeneity in other bulk high-temperature superconductors.