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The origin of anomalously high melting temperatures in gallium clusters

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Small clusters of gallium at sizes ranging from 17 to 85 atoms have been experimentally shown to melt at temperatures hundreds of Kelvin higher than the bulk metal [1], in contravention of the usual parading of melting point depression. On the other hand, the low melting temperature of bulk gallium remains a poorly understood phenomena, with contradictory analyses of the liquid state complicating our understanding. In our previous work [2] we have demonstrated the ability of density functional theory calculations to reproduce the experimental findings, in particular the strong size-sensitivity of the melting temperatures. We have also discussed in depth the rich potential energy landscape of the clusters, in particular the transitions between different structural classes [3]. In this work we have analysed the liquid state of the clusters, and found that in contrast to the usual expectations of spherical drop-like behaviour, the clusters are distorted trivially for large proportions of the simulation time. The shortest axis of the clusters remains consistently at a length suggestive of a bilayer structure, albeit one in which the individual atoms retain significant mobility in the two dimensional plane. These results explain the higher melting temperatures of the clusters as being due to lowered entropy of the liquid phase at small sizes [4], and give insight into important questions yet to be asked about the nature of the bulk liquid.

[1] G. A. Breaux, B. Cao, and M. F. Jarrold. J. Phys. Chem. B, 109 , 16575–16578, 2005; G. Breaux, R. Benirschke, T. Sugai, B. Kinnear, and M. Jarrold. Phys. Rev. Lett., 91 , 215508, 2003. [2] K. G. Steenbergen and N. Gaston. Phys. Rev. B, 88 , 161402, 2013; K. G. Steenbergen and N. Gaston. Phys. Chem. Chem. Phys., 15 , 15325, 2013; K. G. Steenbergen, D. Schebarchov, and N. Gaston. J. Chem. Phys., 137 , 144307, 2012. [3] K. G. Steenbergen and N. Gaston. Chem. – Eur. J. 21 , 2862–2869, 2015. [4] K. G. Steenbergen and N. Gaston. Nano Lett., 16 , 21, 2016.

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