Effect of isovalent and non-isovalent substitutions on superconductivity in NdBa2−xPrxCu3O7

We have compared for the first time the structural and superconducting properties of (A) NdBa2−xPrxCu3O7 and (B) Nd1−uPruBa2Cu3O7. An orthorhombic to tetragonal transition was found in A for x > 0.2 whereas for B no such transition has been reproted. In both cases superconductivity was destroyed for x = 0.3 and u = 0.3 and could be restored by isovalent Sr substitution. However, Ca substitution at the Nd site restored superconductivity only in the case of A. Our results indicate that different mechanisms have to be invoked to account for the suppression of superconductivity in compounds A and B.