Small sodium clusters that melt gradually: Melting mechanisms in Na30

Abstract

The meltinglike transition of Na30 is studied by orbital-free density-functional molecular dynamics simulations. The potential energy surface of Na30 is sampled by simulated annealing and regular quenchings performed along the dynamical trajectories. Both the ground-state structure and low-energy structural excitations are found to exhibit substantial polyicosahedral ordering. The most relevant feature of the potential energy landscape for the melting problem is the existence of many different structural isomers within an energy range of 1 meV/atom, resembling that of a glassy system (yet the structures have a high symmetry). The liquid phase is accessed gradually, with some isomerizations observed at a temperature as low as 30 K, while melting can be considered complete above approximately 200 K. The different dynamical mechanisms that allow the smooth opening of phase space available to the system as a function of temperature are identified and discussed. They can be classified in two different categories: (a) those that allow the exploration of isomers similar to the ground state, involving mainly surface isomerizations and surface melting, and leaving the structure of the cluster core unchanged; and (b) those associated with a more substantial structural change, more similar to the usual solid-solid phase transition in bulk phases; the structure of the cluster core changes only in this second type of transition. Mechanism (a) results in surface melting of the corresponding isomer upon heating; at that stage, mechanism (b) acts to transfer some excess energy from the surface to the core region, so that the surface melting is transiently avoided. Even in the fully developed liquid state, there are important differences from the bulk liquid due to the presence of the surface.