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.