Rocksalt CrC formation on the Fe-15% Cr(100) surface - art. no. 115427

We have used medium-energy ion scattering to investigate segregation of Cr and impurity (20 ppm) C atoms to the surface of Fe-15% Cr(100). Nonequilibr ium cosegregation (surface nucleated precipitation) of Cr and C leads to fo rmation of a CrC surface compound with a structure related to cubic rocksal t. Such a phase does not exist in the bulk, but is stabilized by epitaxy to the bcc(100) surface. The formation of this surface compound (with C conte nt between 1.6 and 2.4 ML) can be viewed as a structural transformation com parable to a giant expansion (approximate to 46%) between the first two met al layers of the alloy surface. Details of the surface composition and stru cture have been obtained on a layer-by-layer basis. C atoms in the first tw o layers are found to be buckled towards the vacuum by approximate to0.2 An gstrom, characteristic of ionic bonding in which the anion has the higher p olarizability. It is suggested that changes in the subsurface carbon concen tration could allow for the controlled engineering of thin-film characteris tics. such as magnetic properties, by modification of near-surface interlay er spacings.