Direct observation of nanometer-scale amorphous layers and oxide   crystallites at grain boundaries in polycrystalline Sr1-xKxFe2As2   superconductors
  We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystalline Sr0.6K0.4Fe2As2 superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale particles sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films. 
Lei Wang, Yanwei Ma, Qingxiao Wang, Kun Li and Xixiang Zhang, Yanpeng   Qi, Zhaoshun Gao, Xianping Zhang, Dongliang Wang, Ch...
cond-mat.supr-con