Hot and Solid Gallium Clusters: Too Small to Melt

Abstract

A novel multicollision induced dissociation scheme is employed to determine the energy content for mass-selected gallium cluster ions as a function of their temperature. Measurements were performed for Ga(+)(n) (n=17 39, and 40) over a 90-720 K temperature range. For Ga+39 and Ga+40 a broad maximum in the heat capacity-a signature of a melting transition for a small cluster-occurs at around 550 K. Thus small gallium clusters melt at substantially above the 302.9 K melting point of bulk gallium, in conflict with expectations that they will remain liquid to below 150 K. No melting transition is observed for Ga+17.

Size-sensitive melting characteristics of gallium clusters: Comparison of experiment and theory forGa17+andGa20

“…17,18 In such cases, we expect that the specific-heats should change from showing a well-defined peak to a rather broad one, and back again to well-defined. We believe this behavior to be generic as it has not only been observed in case of gallium clusters 8,9 but also in case of aluminum clusters 10 , experimentally, and for sodium clusters in the simulations mentioned above.…”

“…Coming to the present work on gallium clusters, it is by now well known that gallium clusters not only melt at substantially higher temperatures than the bulk (T m[bulk] = 303 K), 8 but they also exhibit wide variations in the temperature dependences of their specific-heats, with some clusters showing strong peaks (due to the latent heat), while others (apparent "non-melters") showing no peak. 9 .…”

“…Experimental studies of the melting transitions of clusters in small size regime have only recently become possible. 3,4,5,6,7,8,9,10,11 Several interesting phenomena have been observed, including melting temperatures that rise above the bulk value 7,8 and strong size-dependent variations in the melting temperatures. 9,10 .…”

“…Prior experimental results for Ga + 17 over a limited temperature range showed no evidence for a melting transition. 8 The experimental results in this case were specific-heat measurements performed using multi-collision induced dissociation, where a peak in the specific-heat due to the latent heat was the signature of melting. On the other hand, recent simulations for Ga 17 show a broad peak in the specific-heat centered around 600 K. The previous specific-heat measurements for Ga + 17 extended only up to 700 K, so one possible explanation for this apparent discrepancy is that the melting transition occurred at a slightly higher temperature than examined in the experiments.…”

Cluster melting: new, limiting, and liminal phenomena

“…In 2003, a follow up work from the same group was published, demonstrating that gallium clusters, too, could be made 'too small to melt' [65]. The initial work on cationic clusters of 17, 39, and 40 atoms was followed by a systematic study of the 29-50 atom size range, from which the effects of addition or removal of a single atom became apparent [66].…”