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

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+yTe, Fe_1+yTe_0.8Se_0.2, and Fe_1+yTe_0.5Se_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+yTe. Under these specific structural conditions, interstitial iron atoms interact with adjacent Fe atoms, forming iron polycomplexes that induce pronounced distortions in the FeTe₄ 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+yTe_0.5Se_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+yTe_1-xSe_x crystals.