Surface and Subsurface Segregation of Co Antisite Atoms in CoAl

In order to minimize the total energy of a disordered alloy crystal, those atoms segregate to the surface, which own the lower surface energy. In ordered systems, this energy gain has to be paid by the energy necessary to create antisite atoms in the bulk and at the surface. In strongly ordering systems, i.e. such with a large formation enthalpy, this energetic balance turns out to be unfavourable and hence, no segregation is observed in the ideal stoichiometric case. However, the situation changes completely, when the crystal is even only slightly offstoichiometric. Then, antisites atoms are already present in the bulk and may in fact lower the energy by accumulation at the surface, sometimes even at the expense of replacing atoms with lower surface energy. In that sense, this type of “antisite segregation” is a reversal of the “regular” segregation process. It has recently been postulated on theoretical grounds [1] and independently found by LEED experiments and quantitative analyses for a CoAl(100) surface and explained on the basis of DFT calculations [2].