Geometric Size Effects in the Melting of Sodium Clusters

Melting of neutral and singly charged sodium clusters Naₙ and Naₙ⁺ is investigated for several sizes (8≤n≤139) using both Monte Carlo and molecular dynamics simulations with an empirical many-body potential. Up to n0ex0ex=0ex0ex75, we find that the solid-liquid-like phase change is a multistage process initiated by ``premelting.'' The equilibrium structure is found to play an important role in the thermodynamics, in particular when such premelting features are induced by diffusion processes at the surface. However, larger sizes n≥93 exhibit preferentially a single process similar to bulk melting.