of the surface structure during the heating and cooling processes, phase transitions to a faceted phases, phase diagram, LEED patterns and temperature dependence of diffracted intensity calculated in kinematic approximation.

The SOS model {#s2}
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To study faceting in bimetalic system one needs to know interaction potentials. There are many-body potential for Mo-Mo and W-W interactions derived by Finnis and Sinclair [@finnis] but to our knowledge there is only Ni-Mo many-body potential [@zhang98] for interaction between (Mo, W) and fcc metals. However in Ni/Mo(111) system the faceting does not occur [@mad96]. Therefore we are going to study overlayer induced faceting using a simple solid-on-solid (SOS) model which describe a surface formation energy in agreement with result of the first principle calculation.

In order to construct a model of overlayer-induced faceting on bcc(111) surface (BCCSOS) we are taking into account some experimental evidences. There is a critical coverage, approximately equal to 1PML, to induce faceting on W(111) and Mo(111) [@mad94]. When the coverage exceeds 1PML, extra adsorbate atoms form 3D clusters on the top of faceted surfaces. Hence to simplify problem it will be assumed that coverage is constant and equal to 1PML. Such coverage is equivalent to 3, 2, and 