On the surface composition of intermetallic compounds: The case of MgNi₂

Basic factors governing surface segregation in "size-factor" intermetallics are introduced and applied to the case of Mg segregation in MgNi₂ thin films reported recently. The calculations, based on a Braggs-Williams-type approximation, are in agreement with the experimental results, and indicate a preferential population of surface Ni antisites by the larger Mg atoms from the nearby underlayers. The resultant increase in the equilibrium concentration of Mg with temperature is associated with entropy-driven enhancement of compositional disorder at the surface region of the intermetallic compound.