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Two-dimensional Iron Monocarbide with Planar Hypercoordinate Iron and Carbon

We report on the theoretical discovery of Iron monocarbide binary sheets stabilized at two-dimensional confined space, which we call tetragonal-FeC (t-FeC) and orthorhombic-FeC (o-FeC), respectively. From the energy viewpoint, the proposed t-FeC is the global minimum configuration in the 2D space, and each carbon atom is four-coordinated with ambient four Iron atoms. Strikingly, the o-FeC monolayer is an orthorhombic phase with planar pentacoordinate carbon moiety and planar seven-coordinate Fe moiety. To our knowledge, this monolayer is the first example of a simultaneously pentacoordinate carbon and planar seven-coordinate Fe-containing material. State-of-the-art theoretical calculations confirm that all these monolayers have significantly dynamic, mechanical, and thermal stabilities. Among these two monolayers, t-FeC monolayer shows a higher theoretical capacity (395 mAh g-1 ), and can stably adsorb Li up to t-FeCLi3 . Low migration energy barrier is predicted as small as 0.26 eV for Li, which result in the fast diffusion of Li atom on this monolayer. Moreover, electron-phonon calculations coupled with Bardeen-Cooper-Schrieffer arguments suggest t-FeC can be potential two-dimensional superconductors with 6.77 K superconducting transition temperature.