ADATOMS AND VACANCIES ON A_3B(001) SURFACES

摘要

We have investigated the vibrational and diffusive properties of adatoms and vacancies on the (001) surfaces of Cu_3Au and Ni_3Al alloys us-ing molecular dynamics. For both materials, we found that the low-energy vibrations of the clean ordered surface and of adatoms are due mainly to the heavier atom and that the surfaces are rumpled. We also found that the behavior of the adatoms is marked by the availability of two equilibrium positions: one over the four-fold site and another one on top of a Cu (or Ni) surface atom. The Cu adatom on Cu_3Au(001) is found to diffuse mainly via a new hopping mechanism between neighboring four-fold sites, passing over the on-top adatom position, and a new exchange mechanism involving second-layer atoms. The Au adatom is unstable on Cu_3Au(001), segregating into the surface layer via an exchange mechanism similar to that of the Cu adatom, resulting in loss of local surface order. On Ni_3Al(001), both adatoms are stable and show considerable diffusion with a variety of hopping mechanisms, but exchange-type mechanisms are absent. For both alloys surfaces, vacancy diffusion is favored in the [110] in-plane direction. On Cu_3Au(001), this process results in new stable atomic re-arrangements, with the surface atoms at anti-sites, while for Ni_3Al(001), vacancy hops lead the system to unstable atomic conformations. Thus, the presence of adatoms or vacancies on Ni_3Al(001) does not affect surface order, at least at relatively low temperatures, while on Cu_3Au(001), they stimulate phenomena that are related to the order-disorder transition at higher temperatures.