DIFFERENCE IN SURFACE MELTING BETWEEN INDIUM (110) AND (011)

Medium-energy ion scattering measurements and Monte Carlo computer sim ulations have been used to study the melting behaviour of the two open low-index surfaces of indium: In(110) and In(011). Because of the tet ragonal lattice structure, the atomic density at the Ln(110) surface i s 3.5% lower than that at the In(011) surface. The corresponding highe r surface free energy of the (110) surface is expected to give rise to a slightly stronger surface melting effect at this face than at the ( 011) face. The ion-scattering measurements indeed show such a differen ce in melted-layer thickness at temperatures close to the bulk melting point, T-m=429.76 K. Surprisingly, however, the order of the onset of surface disordering is reversed. In(110) starts disordering about 25 K closer to T-m than In(011). We attribute this counter-intuitive diff erence in disordering onset to the difference in adatom-vacancy creati on energies at the two surfaces. This idea is corroborated by energy c alculations and Monte Carlo computer simulations, in which a Finnis-Si nclair interaction potential between indium atoms was employed. The sl ightly increased nearest-neighbour distance on the more densely-packed In(011) surface leads to a decreased creation energy of adatoms and v acancies. The resulting higher densities of adatoms and vacancies make In(011) unstable at a lower temperature than In(110). These results s trongly suggest that the onset of surface disordering involves a mecha nical surface instability.