Unexpected profiles of surface segregation vibrational entropies

Profiles of vibrational entropy of segregation are calculated as a function of distance from (0 0 1) and (3 1 0) free surfaces for the infinitely dilute Cu(Ag) system. The local vibrational densities of states are evaluated using the recursion method with the force-constant matrix generated from a N-body interatomic potential based on the second moment approximation of the tight-binding scheme. We show that the maximum of the segregation vibrational entropy is not located in the surface plane but in the first underlayer that recovers the bulk coordination number of nearest neighbours. Furthermore, we show that surface relaxations are essential to obtain this unexpected vibrational entropy profile. For the (3 1 0) surface, the maximum value of the segregation vibrational entropy is located on the third underlayer and is larger than for the (0 0 1) orientation. Moreover, we estimate the change in the segregation entropy due to the variation of the mean atomic positions with temperature.