Iron carbide formation on thin iron films grown on Cu(100): FCC iron stabilized by a stable surface carbide

[Display omitted] •Ethylene decomposition produces a pure iron carbide on Fe/Cu(100).•Carbon forms a very stable clock-reconstructed p4g-Fe2C film on FCC-Fe(100).•p4g-Fe2C traps carbon on the surface so that it does not diffuse into the bulk.•p4g-Fe2C surface carbide stabilizes the Fe film as γ-Fe (FCC).•Carbon on BCC-Fe diffuses to the bulk and forms a different carbide. Thin iron films evaporated onto Cu(100) were carburized using ethylene to produce iron carbide surfaces for use as model systems in experimental research. XPS and AES confirm that ethylene dissociation produces a pure iron carbide. A maximum of 0.5 ML carbon can be deposited for film thicknesses below 12 ML where Fe grows as γ-iron (FCC). For thick, BCC-Fe(110) films, post-treatment with ethylene leads to carbon coverages beyond 0.5 ML where some carbon diffuses into the bulk. The film remains α-iron (BCC) and a different surface carbide with a (4 × 3) unit cell is found. On the thin FCC-Fe(100) films, carbon reconstructs the surface into a p4g(2 × 2)-Fe2C layer which has a special stability and acts as a carbon trap that prevents carbon diffusion into the bulk. Fe2C is thermally stable up to 700 K above which Fe diffuses into the copper substrate while leaving graphitic carbon behind. Carbon segregates to the surface during evaporation of iron on top of an Fe2C-covered FCC-Fe film and causes the film to retain the FCC structure up to a thickness of at least 30 ML, far beyond 12 ML where BCC-Fe forms on Cu(100) in absence of surface carbon.