Melting transitions in aluminum clusters: The role of partially melted intermediates

Abstract

Heat capacities have been measured as a function of temperature for aluminum cluster cations with 16–48 atoms. Some clusters show peaks in their heat capacities that are attributed to melting transitions. The smallest cluster to show a well-defined melting transition is $\mathrm{Al}_{28}{}^{+}$. For clusters with significant peaks in their heat capacities, the results can be fitted by a two-state model incorporating only solidlike and liquidlike clusters. This indicates that these clusters melt directly, that is, without the involvement of partially melted intermediates. Our previously reported heat capacity measurements for clusters with 49–83 atoms have been reanalyzed using the two-state model and a three-state model that incorporates an intermediate state. Most of the melting transitions can be fitted using the two-state model. However, for a few clusters, the heat capacity peaks are either too broad or possess shoulders, and the three-state model is required to fit the experimental results. Both premelting and “postmelting” behaviors (where the second peak is smaller than the first) are observed. Using the models, we have determined melting temperatures and latent heats for clusters with 25–83 atoms. The melting temperatures and latent heats show large (and uncorrelated) size-dependent fluctuations. While most clusters have depressed melting temperatures, there are three regions of high melting temperatures (around 37, 47, and 66 atoms) where the melting temperatures approach or exceed the bulk melting point.