Heat capacities have been determined for unsupported aluminum clusters, $\mathrm{Al}_{49}{}^{+}–\mathrm{Al}_{63}{}^{+}$, from 150 to 1050 K. Peaks in the heat capacities due to melting occur between 450 and 650 K (well below the bulk melting point of 933 K). The peaks for $\mathrm{Al}_{51}{}^{+}$ and $\mathrm{Al}_{52}{}^{+}$ are bimodal, suggesting the presence of a premelting transition where the surface of the clusters melts around 100 K before the core. For clusters with $n>55$ the melting temperatures suddenly drop, and there is a dip in the heat capacities due to a transition between two solid forms before the clusters melt.

• Received 30 November 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.173401