How far away are iron carbide clusters from the bulk?

Combining the basin hopping structure searching algorithm and density functional theory, the iron carbide clusters, Fe_xC_y (x ≤ 8 and y ≤ 8), and clusters with various stoichiometries (Fe_2nC_n, Fe_3nC_n, Fe_nC_2n, Fe_nC_3n and Fe_nC_4n (n = 1-7), Fe_5nC_2n, and Fe_4nC_n (n = 1-5)) are predicted. The stable structures of iron rich carbide clusters are composed of C-C dimers or single C atoms on the surface of the clusters, which are remarkably different from their corresponding bulk structures, where the carbon atoms are atomically distributed in the iron matrix. The most stable carbon rich clusters are highly diverse in topology (bowl, basket, plane, shoe, necklace, etc.) with long carbon chains. The Bader charge analysis shows that the size effect on iron carbide clusters is an electronic tuning. Large carbon-rich clusters appear even under low carbon chemical potentials, whereas small iron-rich clusters are only energetically stable in high carbon chemical potentials, which indicates that changing the carbon chemical potential can tune the morphology (size and stoichiometry) of the iron carbide clusters. These results may help us understand the catalytic activity of iron and iron carbides in reactions such as the Fischer-Tropsch synthesis and the carbon nanotube formation process.