Ab initio molecular dynamical investigation of the finite temperature behavior of the tetrahedral Au-19 and Au-20 clusters

作者：Krishnamurty Sailaja; Shafai Ghazal S; Kanhere D G^*; de Bas B Soule; Ford M J

来源：Journal of Physical Chemistry A, 2007, 111(42): 10769-10775.

DOI：10.1021/jp075896+

Density functional molecular dynamics simulations have been carried out to understand the finite temperature behavior of Au-19 and Au-20 clusters. Au-20 has been reported to be a unique molecule having tetrahedral geometry, a large HOMO-LUMO energy gap, and an atomic packing similar to that of the bulk gold (Li, J.; et al. Science 2003, 299, 864). Our results show that the geometry of Au-19 is exactly identical with that of Au-20 with one missing corner atom (called a vacancy). Surprisingly, our calculated heat capacities for this nearly identical pair of gold clusters exhibit dramatic differences. Au-20 undergoes a clear and distinct solid-like to liquid-like transition with a sharp peak in the heat capacity curve around 770 K. On the other hand, Au-19 has a broad and flat heat capacity curve with continuous melting transition. This continuous melting transition turns out to be a consequence of a process involving a series of atomic rearrangements along the surface to fill in the missing corner atom. This results in a restricted diffusive motion of atoms along the surface of Au-19 between 650 to 900 K during which the shape of the ground state geometry is retained. In contrast, the tetrahedral structure of Au-20 is destroyed around 800 K, and the cluster is clearly in a liquid-like state above 1000 K. Thus, this work clearly demonstrates that (i) the gold clusters exhibit size sensitive variations in the heat capacity curves and (ii) the broad and continuous melting transition in a cluster, a feature that has so far been attributed to the disorder or absence of symmetry in the system, can also be a consequence of a defect (absence of a cap atom) in the structure.

• 出版日期2007-10-25