Special grain boundaries in high-Tc Bi2Sr2CaCu2O8+δ

Abstract

Abstract The structure of grain boundaries of highly dense Bi 2 Sr 2 CaCu 2 O 8+δ was investigated by high-resolution transmission electron microscopy. Special grain boundaries were observed for which the c -axes of adjacent grains were parallel to each other. Grains which are rotated with respect to each other by approximately 45° around the c -axis form coherent (001) grain boundaries. For (001) twin boundaries the rotation angle is 90° around the c -axis. For coherent (001) grain boundaries and twin boundaries, the interfaces lie between two adjacent (001) BiO layers. This indicates that the formation of (001) interfaces between these two layers is energetically most favourable due to the large bond lengths resulting in weak chemical bonds. Polytypoids, i.e. lamellae of Bi 2 Sr 2 Ca n −1 Cu n O 4+2 n +δ phases with ≠2, are frequently observed at twin boundaries and also occur at small angle tilt boundaries to accommodate for the mismatch. This yields compositional variations along the boundary. The dislocations in the boundary introduce either additional perovskite layers or Bi-O double layers. The latter dislocations occur less frequently and convert one lamella with n =3 or n =4 into two lamellae of n =1 phase. Incoherent grain boundaries show no evidence of intergranular phases. Thus, the weak-link behaviour of grain boundaries, which is observed in YBa 2 Cu 3 O 7− x might not occur in Bi 2 Sr 2 CaCu 2 O 8+δ ceramics.