Electronic Shell and Dynamical Coexistence Effects in the Melting of Aluminum Clusters: An Interpretation of the Calorimetric Experiments Through Computer Simulation

We report first-principles computer simulations of the melting-like transition in AlnQ (n = 35–37, Q = +1,0, −1) clusters. Melting induces an abrupt modification of the electronic shell structure, whereby the HOMO–LUMO gap and chemical hardness sharply decrease, and the size dependence of electronic properties becomes smoother. This intimate coupling between electronic and thermodynamic properties unveils an explicit electronic contribution to the latent heat and other melting properties. Dynamical coexistence involves the participation of a “hot solid” plastic phase, characterized by cooperative atomic rearrangements that do not destroy the structural order. The hot solid acts as a buffer thermodynamic phase connecting the solid and liquid states, and may lead to a double peak structure in the microcanonical heat capacities. A negative microcanonical heat capacity is predicted for neutral Al36. Finally, the average radial atomic densities of liquid clusters show layered profiles that lie on a universal curve if appropriately scaled variables are defined. The layering is interpreted as a premonitory freezing effect, which may have practical implications concerning the prediction of freezing points based purely on properties of the liquid phase.