Origin of grain boundary weak links in BaPb1−xBixO3 superconductor

Abstract

Although BaPb0.75Bi0.25O3 (BPB) has a comparatively large superconducting coherence length of ∼7 nm and no reported anisotropy in its superconducting parameters, polycrystalline BPB exhibits the same rapid decrease in transport critical current density (Jct) with low applied field (<∼50 Oe) that is characteristic of grain boundary weak links in cuprate superconductors (e.g., La2−xSrxCuO4, YBa2Cu3O7−x ). We have studied the effects of processing thermal history on the formation and morphology of grain boundary phases, and on the composition of BPB boundaries with and without second phase, in order to understand the origin of these weak links. Scanning transmission electron microscopy and Auger electron spectroscopy results show the presence of a Pb-Bi-Ba-O phase that is wetting and liquid above ∼570 °C, but which retracts to three-grain junctions upon slow cooling or annealing at lower temperatures. However, weak-link behavior persists in materials with retracted secondary phase, as well as in hot isostatically pressed samples that never exceed the secondary phase melting temperature. It is found that the grain boundaries remain Bi- and Pb-rich even after the retraction of secondary phases; samples that never exceed the melting temperature of the secondary phase show absence of segregation at some but not all grain boundaries. The composition of the grain boundaries as well as Jct vs temperature measurements indicate that the boundaries act as SIS tunnel junctions.