Molecular dynamics study of the ordered Cu 3Au : II. Vibrational and structural properties of Cu and Au adatoms on the low indexed surfaces

Abstract

Using an effective potential model in analogy to the tight-binding scheme to the second-moment approximation, we investigated the vibrational properties of the Cu and Au adatoms on the low index ordered Cu 3Au surfaces. We found that the presence of adatoms alters the surface phonon modes and introduces new ones situated mainly at high frequencies. This result indicates that the coupling between adatoms and surface atoms is stronger than between surface atoms. Moreover, we found that both adatoms on the (1 1 1) faces are unstable, segregating already at room temperature. On the (0 0 1) surface the vibrational amplitudes of both adatoms depend linearly on temperature up to T s=500 K, exhibiting anomalous increase above this temperature. Concerning the (1 1 0) surface, the mean-square-displacements and the relaxed interlayer positions of the adatoms present strong anharmonic behaviour, the effect being more pronounced above T s, accompanied by segregation and spontaneous creation of adatoms. Therefore, T s appears as a characteristic temperature, above which phenomena stimulated by the presence of adatoms set up acting as precursors of the order–disorder transition which happens around T r=663 K for the bulk system. These findings are compatible with available experimental and theoretical data and with results referring to Au deposition on the low indexed Cu faces.