  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.
