MAM modelling of the segregation and ordering at the Pt3Sn(111) surface

The temperature dependent segregation and ordering phenomena at the Pt3Sn(1 11) surface are modelled by means of Monte Carlo simulations combined with the "Macroscopic Atom" Model (MAM). The results are compared with experimen tal data by Ceelen et al. [W.C.A.N. Ceelen, A.W. Denier van der Gon, M.A. R eijme, H.H. Brongersma, I. Spolveri, A. Atrei, U. Bardi, Surf. Sci. 406 (19 98) 264]. It appears from the simulations that the experimentally observed (root 3 x root 3)R30 degrees structure at 700 K in the increasing temperatu re trajectory is a consequence of two factors: a preferential sputtering in the cleaning procedure and a limited atomic mobility, at lower temperature s restricted to the two outermost layers. It is thus induced by the sample preparation and hence merely an artefact rather than an intrinsic property. Only for temperatures between 1000 and 1200 K is full equilibrium between surface and bulk reached. The endothermic Sn segregation and corresponding (2 x 2) order at these temperatures are also evidenced in the MAM calculati ons that are in good agreement with the experimental data. (C) 2000 Elsevie r Science B.V. All rights reserved.