Theoretical study of the melting of aluminum clusters

The melting of Al clusters in the size range $49⩽N⩽62$ has been studied by computer simulation in order to help interpret recent experimental results in this size range. Two model interatomic potentials are used: a Gupta potential and a glue potential which favor icosahedral and polytetrahedral structures, respectively. The glue potential exhibits a smooth relatively featureless heat capacity curve for all sizes except for $N=54$ and $N=55$, sizes at which icosahedral structures are favored over the polytetrahedral. Gupta heat capacity curves, instead, show a well-defined peak that is indicative of a first-order-like transition. The differences between the two models reflect the different ground-state structures, and neither potential is able to reproduce or explain the experimentally observed size dependence of the melting transition, hence suggesting that the experimental structures are unlikely to be icosahedral or polytetrahedral.

Figure 1

Comparison of $\varphi \left(r\right)$, $\rho \left(r\right)$, and $F\left(\overline{\rho }\right)$ for Al Gupta and glue potentials in the effective pair format.

Figure 2

(Color online) Ground state structures for Al (a) Gupta and (b) glue clusters with 49 to 62 atoms. For the 61-atom glue cluster, the disclination network is also depicted.

Figure 3

Heat capacities as a function of temperature for aluminum clusters from 49 to 62 atoms, predicted by the glue (solid line) and Gupta (dashed line) models.

Figure 4

Comparison of (a) the melting temperatures and (b) latent heats of fusion and entropies of fusion (dashed lines) as calculated with the Gupta model (closed circles) with the values obtained experimentally (open circles). In (b) the quantities are measured as a fraction of their bulk values.

Figure 5

A structural analysis of the melting of the (a) Gupta ${\mathrm{Al}}_{58}$ and (b) glue ${\mathrm{Al}}_{56}$ clusters. In (a) the percentage of quenches leading to the icosahedral ground-state structure (closed circles), to structures which have absorbed some or all the extra atoms in the surface (diamonds), and to high energy isomers (open circles) is plotted. In (b) the structures considered are the polytetrahedral ground-state structure (closed circles), an icosahedral isomer (diamonds), and high energy isomers (open circles).