Distinct Diffusion and Recombination Dynamics of Atomic Carbon and Oxygen on Face-Centered Cubic and Body-Centered Cubic Iron Multilayers

This study investigates the distinctly different dynamics of atomic carbon and oxygen diffusion, both on the surface and into the bulk of iron multilayer films with face-centered cubic (FCC) (100) and body-centered cubic (BCC) (110) structures, and how these processes impact the recombination behavior of carbon and oxygen, particularly at elevated temperatures. On FCC-iron (y-iron), CO dissociation occurs around 300 K, leading to the formation of segregated carbide and oxide islands on the surface upon annealing. Above the onset temperature of 600 K, mobile oxygen atoms diffuse to the edge of the carbide islands, where they combine with carbon to form CO. In contrast, on BCC (a-iron) surfaces, a disordered, atomically mixed carbide-oxide phase forms upon CO dissociation. Carbon does not remain on the surface but migrates to the subsurface during heating, leaving oxygen on the surface. Carbon remains predominantly subsurface following CO dissociation, enabling a direct recombination pathway between subsurface carbon and surface oxygen. This subsurface activity requires lower activation, resulting in CO recombination and then desorption at lower temperatures compared to the FCC system. These distinct pathways observed on y-FCC and a-BCC iron surfaces have significant implications for materials science, metallurgy, and catalysis, highlighting the critical role of thermodynamic and kinetic factors in governing atomic diffusion and recombination processes.