Revealing the complex chemistry of grain boundaries in K-doped BaFe2As2 with atom probe tomography

• A computational method was developed to extract the Gibbsian interfacial excess • The validity of the method was tested using simulated atom probe tomography volumes • This method was used to analyse grain boundaries in K-doped Ba-122 superconductors • Segregation of As, Ba, K and impurities of O, Na, and Sb were found at boundaries • The link between the microstructure and the critical current density was explored Iron-based superconductors have attractive properties for high-field applications, but there is a lack of understanding of the effect of grain boundary chemistry on the in-field performance. The near atomic-scale resolution, ppm sensitivity and 3D analysis offered by atom probe tomography make it a powerful tool to investigate the nanoscale structure and chemistry of these defects in fine-grained K-doped BaFe 2 As 2 samples. A computational method to systematically extract and compare the Gibbsian interfacial excess of chemical species across grain boundaries has been explored in this work. The robustness of the method has been tested by evaluating the effects of selected variables on simulated APT datasets. The accuracy and precision of the calculated Gibbsian interfacial excess were found to be stable over a range of analysis conditions: varying grain boundary widths and detection efficiencies, spatial precisions below 1.5 nm, and bin widths between 1.2 and 1.6 nm. For the K-doped BaFe 2 As 2 samples studied, segregation of As, Ba, K and impurities of O, Na, and Sb were found at grain boundaries. The Gibbsian excess values were found to vary widely between different boundaries, showing the complexity of the grain boundary chemistry in this material. Possible links between the observed critical current density ( J c ) of these samples and their nano- and micro-structure have also been investigated and discussed.