Molecular dynamics study of the transport and structural properties of the Cu3Au and Ni3Al(1 1 0) surface

We present molecular dynamics results concerning the transport and the structural properties of the Cu3Au and Ni3Al(1 1 0) surfaces in presence of adatoms and vacancies. In the Cu3Au case we found that below 500 K the Cu adatom occupies preferably the so-called “dumb-bell” position, while the Au adatom penetrates into the second pure Cu layer via an exchange type mechanism inducing thereby local disorder. At higher temperatures the Cu adatom resides almost exclusively at fourfold positions, while spontaneous creation of adatom–vacancy pairs is also present. Vacancy diffusion processes induce surface disorder that starts by as much as 150 K below the bulk order–disorder temperature. In the case of Ni3Al(1 1 0) surface, we found a new Al adatom position situated between two Ni surface atoms. It came out that this position plays an important role in the adatom’s diffusive behavior. Above 800 K adatom–vacancy pairs are spontaneously created and in conjunction with exchange type diffusion mechanisms they affect seriously the surface order. In addition, we found that the Ni adatoms are more active than the Al adatoms, this difference in the adatom diffusivity being accentuated by exchange events, leading very quickly the surface in a disordered state. In conclusion, it comes out that the Cu3Au and Ni3Al(1 1 0) surfaces disorder well below the bulk transition temperature (663 K, Cu3Au) and the melting point (1663 K, Ni3Al) via diffusion processes of the adatoms and/or vacancies.