Origin of the Diverse Melting Behaviors of Intermediate-Size Nanoclusters: Theoretical Study of AlN (N = 51−58, 64)

Microscopic understanding of thermal behaviors of metal nanoparticles is important for nanoscale catalysis and thermal energy storage applications. However, it is a challenge to obtain a structural interpretation at the atomic level from measured thermodynamic quantities such as heat capacity. Using first-principles molecular dynamics simulations, we reproduce the size-sensitive heat capacities of Al(N) clusters with N around 55, which exhibit several distinctive shapes associated with diverse melting behaviors of the clusters. We reveal a clear correlation of the diverse melting behaviors with cluster core symmetries. For the Al(N) clusters with N = 51-58 and 64, we identify several competing structures with widely different degree of symmetry. 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) → D(2d) → C(s)) 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.