COSEGREGATION-INDUCED FORMATION OF 2-DIMENSIONAL SURFACE-COMPOUNDS ONMULTICOMPONENT ALLOY SURFACES

The chemical composition of multicomponent alloy surfaces may exhibit significant deviations from the bulk composition due to thermally acti vated segregation and cosegregation processes. In many systems cosegre gation phenomena result in the formation of two-dimensional surface co mpounds such as CrN on Fe-15% Cr-N(100), VC on Fe-3% V-C(100) and on F e-3% Si-0.04% V-C(100), and TiC on Fe-6% Al-0.5% Ti-C(100). These surf ace compounds are epitaxially stabilized on the bcc(100) alloy surface s, their thicknesses are about one or two atomic layers. In the presen t work we report results of a Monte Carlo study on the cosegregation-i nduced formation of surface compounds on bcc(100) surfaces. The simula tions are performed utilizing a three-dimensional lattice gas model wi th two free (100) surfaces and periodic boundary conditions in x and y directions. It is assumed that the lattice consists of two types of l attice sites M and X. The metal sites M form a body-centered cubic (bc c) lattice, whose quasi-octahedral interstices constitute the nonmetal sublattice. The M sites are accessible to either M(A) or M(B) atoms, while the nonmetal sites either are occupied by X atoms or remain empt y. Pairwise repulsive nearest and attractive next nearest neighbor int eractions between M(A)-X and M(B)-X atoms are considered as well as up to fourth nearest neighbor X-X repulsions. The simulations indicate t hat cosegregation-induced formation of the surface compound M(B)X is b asically due to preferential next nearest M(B)-X neighbor attractions. With an increase of the strength of the preferential next nearest M(B )-X attractions, surface compound formation is accompanied by a first- order phase transition. Depending on the relative magnitude of the nea rest neighbor M-X repulsions, we observe a strong M(B) subsurface enri chment, which has been verified by XPD for the CrN surface compound on Fe-15% Cr-N(100). For repulsive fourth nearest X-X neighbor interacti ons, our lattice gas model shows c(2x2) ordering of the X atoms on the bcc(100) surface.