Effect of chemical inhomogeneity in bismuth-based copper oxide superconductors

We examine the effect on the superconducting transition temperature (T c ) of chemical inhomogeneities in Bi 2 Sr 2 CuO 6 + δ and Bi 2 Sr 2 CaCu 2 O 8 + δ single crystals. Cation disorder at the Sr crystallographic site is inherent in these materials and strongly affects the value of T c . Partial substitution of Sr by Ln (Ln = La, Pr, Nd, Sm, Eu, Gd, and Bi) in Bi 2 Sr 1 . 6 Ln 0 . 4 CuO 6 + δ results in a monotonic decrease of T c with increasing ionic radius mismatch. By minimizing Sr site disorder at the expense of Ca site disorder, we demonstrate that the T c of Bi 2 Sr 2 CaCu 2 O 8 + δ can be increased to 96 K. Based on these results we discuss the effects of chemical inhomogeneity in other bulk high-temperature superconductors.