Molecular dynamics study of the ordered Cu3AuII. Vibrational and structural properties of Cu and Au adatoms on the low indexed surfaces

Using an effective potential model in analogy to the tight-binding scheme t o the second-moment approximation, we investigated the vibrational properti es of the Cu and Au adatoms on the low index ordered Cu3Au surfaces. We fou nd that the presence of adatoms alters the surface phonon modes and introdu ces new ones situated mainly at high frequencies. This result indicates tha t the coupling between adatoms and surface atoms is stronger than between s urface 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 u p to T-s = 500 K, exhibiting anomalous increase above this temperature. Con cerning the (1 1 0) surface, the mean-square-displacements and the relaxed interlayer positions of the adatoms present strong anharmonic behaviour, th e effect being more pronounced above T-s, accompanied by segregation and sp ontaneous creation of adatoms. Therefore, T-s appears as a characteristic t emperature, above which phenomena stimulated by the presence of adatoms set up acting as precursors of the order-disorder transition which happens aro und T-r = 663 K for the bulk system. These findings are compatible with ava ilable experimental and theoretical data and with results referring to An d eposition on the low indexed Cu faces. (C) 2001 Elsevier Science B.V. All r ights reserved.