Ordering of Interstitial Iron Atoms and Local Structural Distortion Induced by Iron Polycomplex in Fe _1+y Te _1−x Se _x as Seen via Transmission Electron Microscopy

Abstract The microscopic crystalline structure of materials is widely recognized as having a profound impact on their functional properties and application potential. Alterations to the lattice often provide distinctive opportunities to finely tune specific properties, particularly in strongly correlated systems. A paradigmatic case is the iron‐based high‐temperature superconductors, where the microstructure plays an important role in modulating superconductivity. In this work, aberration‐corrected scanning transmission electron microscopy (STEM) is employed to investigate the microstructure and intrinsic chemical heterogeneity of Fe _1+y Te, Fe _1+y Te _0.8 Se _0.2 , and Fe _1+y Te _0.5 Se _0.5 . A previously unforeseen superstructure phase, characterized by a wave vector q = (0.4, 0, 0.5), arising from the ordered arrangement of interstitial iron atoms, is clearly visible in the parent compound Fe _1+y Te. Under these specific structural conditions, interstitial iron atoms interact with adjacent Fe atoms, forming iron polycomplexes that induce pronounced distortions in the FeTe ₄ tetrahedra and may potentially foster the emergence of ferromagnetic clusters. The experimental findings further illustrate that appropriate Se substitution effectively suppresses interstitial iron concentration and ordering, with Fe _1+y Te _0.5 Se _0.5 notably exhibiting the lowest concentration. The observations also suggest that Se substitution occurs randomly and Te/Se‐induced nanoscale phase separation, driven by chemical heterogeneity is commonly observed within Fe _1+y Te _1−x Se _x crystals.