Emergence of a non-bulk hexagonal Fe$_2$S$_2$ single layer via phase transformation

Affan Safeer; Wejdan Beida; Felix Oberbauer; Nicolae Atodiresei; Gustav Bihlmayer; Max Wolfertz; Chiara Schlichte; Wouter Jolie; Stefan Blügel; Jeison Fischer; Thomas Michely

arXiv · 2604.21613

Emergence of a non-bulk hexagonal Fe$_2$S$_2$ single layer via phase transformation

Affan Safeer·Wejdan Beida·Felix Oberbauer·Nicolae Atodiresei·Gustav Bihlmayer·Max Wolfertz·Chiara Schlichte·Wouter Jolie·Stefan Blügel·Jeison Fischer·Thomas Michely

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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 Fe$_2$S$_2$ single layer with $β$-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 Fe$_2$S$_2$ forms reproducibly upon annealing and represents a new structural motif within the Fe-S material family. First-principles calculations identify the $β$-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 Fe$_2$S$_2$ as a platform for exploring structural polymorphism in two dimensions and demonstrate that reduced dimensionality can stabilize crystal structures not accessible in bulk materials.

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Affan Safeer, Wejdan Beida, Felix Oberbauer, Nicolae Atodiresei, Gustav Bihlmayer, Max Wolfertz, Chiara Schlichte, Wouter Jolie, Stefan Blügel, Jeison Fischer, Thomas Michely. 2026-04-23. Emergence of a non-bulk hexagonal Fe$_2$S$_2$ single layer via phase transformation. https://arxiv.org/abs/2604.21613

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