Geometric size effects in the melting of sodium clusters

Melting of neutral and singly charged sodium clusters Na-n and Na-n(+) is investigated for several sizes (8 less than or equal to n less than or equal to 139) using both Monte Carlo and molecular dynamics simulations with an empirical many-body potential. Up to n = 75, 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 greater than or equal to 93 exhibit preferentially a single process similar to bulk melting.