Unexpected Structures and Properties in a Rare Composition Ratio Two-Dimensional Iron Boride FeB₇

We report the prediction of unexpected structures and properties for a rare composition ratio two-dimensional iron boride FeB7 based on a comprehensive crystal structure search, density functional theory (DFT) calculations, and molecular dynamics simulations. We uncover nine low–lying allotropes of the two-dimensional FeB7 crystals with novel configurations and superior properties. The global minimum structure FeB7–1 and its twin structures FeB7–4 and FeB7–8 are layered structures with symmetrical 12-coordinate Fe-center sandwiches. They all have high structural stabilities. The elastic constant calculations reveal excellent mechanical properties for FeB7–1. The special energy band structures of the low–lying allotropes FeB7–4 and FeB7–8 suggest that they could be applied in future spin-filtering devices. Considering other low-lying allotropes in the 2D FeB7 system, we found that some of these metastable structures have exotic properties. For example, FeB7–5 has a rare negative Poisson’s ratio. We also propose several potential methods for fabricating these novel stable 2D FeB7 nanosheets. Our work will open up new research directions and possibilities for future experimental exploration of 2D iron borides. We hope that this study will provide inspiration and guidance for experimental synthesis of these novel materials.