EQUILIBRIUM SURFACE SEGREGATION OF INTERSTITIALS ON BCC (001) SURFACES

The surface segregation of interstitials X on free (001) surfaces of a body-centered cubic metal lattice M is studied by means of Monte Carl o modeling. It is assumed that interstitially dissolved X atoms occupy irregularly shaped octahedral bulk interstices characteristic of the bcc structure. Pairwise nearest and more distant neighbor couplings be tween adjacent M and X atoms phi(MX)(i) are used to calculate binding energies for X atoms at the various bulk and surface sites. Additional repulsive interactions between adjacent X atoms phi(XX)(i) are consid ered. Monte Carlo simulations are performed for three different sets o f interaction parameters phi(XX)(i). In all these cases the fourfold h ollow sites of the bcc (001) surface are the most stable coordination sites. Ideal behavior in the bulk and at the surface is found if all i nteraction energies phi(XX)(i) = 0. For slightly repulsive nearest and next nearest neighbor interactions phi(XX)(1), phi(XX)(2) < 0 X atoms on the fourfold hollow sites of the surface still behave ideal while deviations from ideal behavior are observed in the bulk for concentrat ions x greater than or similar to 0.01. With additional repulsive four th nearest neighbor interactions phi(XX)(4) < 0 c(2 x 2) ordering is i nduced at the surface for coverages theta almost-equal-to 0.5. In this case interstitials segregated at fourfold hollow sites also display n onideal behavior. The excess Gibbs free energy of segregation DLETAG(S eg)xs is evaluated according to the Langmuir-McLean equation. In the c ase of phi(XX)(1), phi(XX)(2) = 0 we find - DELTAG(Seg)xs = \phi(MX)(1 )\. For phi(XX)(1), phi(XX)2 < 0 and low bulk concentrations x less th an or similar to 0.01 there is still Langmuir-McLean behavior, i.e. -D ELTAG(Seg)xs = \phi(MX)(1)\ = const, but with increasing bulk concentr ations strong positive deviations from ideal behavior are found, -DELT AG(Seg)xs = f(theta, T) > \phi(MX)(1)\. In contrast, negative deviatio ns -DELTAG(Seg)xs < \phi(MX)(1)\ are observed in the case of c(2 x 2) ordered surface phases (phi(XX)(4) < 0), which strongly depends on sur face coverage.