FINITE-TEMPERATURE PROPERTIES OF SODIUM CLUSTERS

We present geometric and energetic properties of sodium clusters with 8, 14, 20, 30, and 40 atoms using an effective many-body interaction among sodium atoms in the framework of an improved isothermal molecular-dynamics approach. These clusters undergo two phase transitions and the two transition temperatures increase with the cluster size. These phase transitions are the equivalents of bulk melting and of boiling in a finite system. However, finite particle effects are particularly strong. These clusters show a more pronounced thermal expansion than the bulk, with significant nonlinear effects. Both the shape and size change rather dramatically with the temperature and the ionic degrees of freedom give a dominating contribution to the entropy, thus effectively controlling the thermal behavior of these clusters.