Melting of gold clusters

Abstract

Investigations of the thermal evolution of structural and dynamic properties of gold nanocrystalline clusters of variable size $({\mathrm{Au}}_{75},$ ${\mathrm{Au}}_{146},$ and ${\mathrm{Au}}_{459})$ were performed using extensive molecular-dynamics simulations employing embedded-atom interactions. These studies reveal that the thermal evolution of these clusters is punctuated by diffusionless solid-to-solid structural transformations from the low-temperature optimal structures (truncated-decahedra for ${\mathrm{Au}}_{75}$ and ${\mathrm{Au}}_{146},$ and a face-centered-cubic structure with a truncated-octahedral morphology for ${\mathrm{Au}}_{459})$ to icosahedral structures. These structural transformations are precursors to the melting transitions which occur at temperatures below the bulk melting temperature of crystalline gold, and they are intrinsic to the thermodynamics of the clusters. The melting scenario revealed by the simulations for these gold clusters differs from that involving thickening of a quasiliquid wetting surface layer, and in addition it does not involve at any stage of the thermal process dynamic coexistence where the cluster fluctuates between being entirely solid or liquid. For the larger cluster, ${\mathrm{Au}}_{459},$ a thermodynamic (icosahedral) solid-liquid coexistence state is found in the vicinity of the melting transition. The occurrence of polymorphic solid structures, that is a cluster containing simultaneously decahedral and icosahedral parts, is discussed in light of early observations of such structures via high-resolution electron microscopy.