Origin of the Diverse Melting Behaviors of Aluminum Nanoclusters with Around 55 Atoms*

Microscopic understanding of thermal behaviors of metal nanoparticles is important for nanoscale catalysis and thermal energy storage applications. Using first-principles molecular dynamics simulations, we reveal the microscopic origin of the diverse melting behaviors of Al$_{N}$ clusters with N around 55 [1,2]. The conceptual link between the degree of symmetry (e.g., T$_{d}$, D$_{2d}$ and C$_{s})$ and solidity of atomic clusters is quantitatively demonstrated through the analysis of the configuration entropy. The size-dependent, diverse melting behaviors of Al clusters originate from the reduced symmetry (T$_{d} \quad \to $ D$_{2d} \quad \to $ Cs) with increasing the cluster size. In particular, the sudden drop of the melting temperature and appearance of the dip at N = 56 are due to the T$_{d}$-to-D$_{2d}$ symmetry change, triggered by the surface saturation of the tetrahedral Al$_{55}$ with the T$_{d}$ symmetry.\\[4pt] [1] G. A. Breaux, C. M. Neal, B. Cao, and M. F. Jarrold, Phys. Rev. Lett. \textbf{94}, 173401 (2005).\\[0pt] [2] J. Kang, S.-H. Wei, and Y.-H. Kim, J. Am. Chem. Soc. (in press).