(111) surface becomes unstable at the coverage of one physical monolayer, where the energy of (211) orientation has the lowest value. However, such energy calculation (at T=0) are not sufficient to explain why some of these metals do not induce faceting, e.g., Ag/Mo(111). On the other hand, the first principle calculation confirmed that critical coverage to induce faceting is approximately equal to one physical monolayer (PML) and that the growth mode is Stransky-Krastanov as the surface energy increases at coverage higher than 1 PML.

Theoretical studies of surface reconstruction and surface phase transitions in bimetalic system are complicated problems mainly due to occurrence of long range many-body interactions. One of approaches to such problems is application of simple solid-on-solid models [@beijeren77; @tosat95; @beijeren95; @kaseno97], in which the crystal is represented by two-dimensional array of columns. They were employed to study roughening transition [@beijeren77; @tosat95], missing-row reconstruction [@tosat96], growth of the surface [@tosat95], surface diffusion [@tosat95; @sieradz95], adsorption [@kaseno97], phase transitions in two component crystal [@beijeren95], etc.

In this paper we introduce a simple SOS model for bcc(111) surface covered by one physical monolayer of adsorbed atoms to describe surface reconstruction and phase transitions. Using Monte Carlo simulation we study change 