Chemically Degraded Grain Boundaries In Fine-Grain Ba0.6k0.4fe2as2 Polycrystalline Bulks

In this study, we investigated nanostructure and local compositional variations at grain boundaries (GBs) in polycrystalline Ba0.6K0.4Fe2As2 (K-Ba122) bulks. Analytical scanning transmission electron microscopy revealed that K-Ba122 bulks made in a regular glove box filled with 99.9% Ar show Ba-O segregation at the GBs, forming a network of insulated GBs. By significantly reducing the oxygen and water content in the glove box, K became preferential segregation at GBs. Our nanostructural study suggests that the K-Ba122 GBs are easily compromised by different kinds of chemistry variations that can be fully addressed only by synthesis in a very clean environment.