Unexpected Structures and Properties in a Rare CompositionRatio 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.