Emergence of a Hexagonal Fe 2 S 2 Single Layer Without a Bulk Counterpart via Phase Transformation

ABSTRACT Two‐dimensional materials can stabilize crystal structures that are absent from their bulk counterparts, offering opportunities for materials design. Here, we report the synthesis of a previously unknown hexagonal single layer with ‐CuI structure, a buckled layer of two vertically stacked FeS honeycomb lattices, realized by thermally induced transformation of single‐layer mackinawite grown on graphene/Ir(111). In situ scanning tunneling microscopy and low‐energy electron diffraction reveal a transition from a tetragonal to a hexagonal lattice accompanied by distinct morphological and electronic signatures. The hexagonal forms reproducibly upon annealing and represents a new structural motif within the Fe–S material family. First‐principles calculations identify the ‐CuI structure as most consistent with experiment. The calculations suggest that on‐site Coulomb interactions and magnetic order are relevant to understanding the stability of the new 2D Fe‐S compound. The preferred nucleation of single‐layer mackinawite, despite being energetically disfavored, is speculated to result from its low edge energy, analogous to the 3D case. Our results establish as a platform for exploring structural polymorphism in two dimensions and demonstrate that reduced dimensionality can stabilize crystal structures not accessible in bulk materials.