Premelting and postmelting in clusters.

Area of Science:

• Physical chemistry
• Materials science
• Computational physics

Background:

• Atomic clusters exhibit unique thermodynamic properties distinct from bulk materials.
• The solid-to-liquid phase transition in small systems is complex and size-dependent.
• Surface melting and core stability are critical factors in cluster phase transitions.

Purpose of the Study:

• To investigate the solid-to-liquid phase transition in sodium clusters of specific sizes (N=139 and N=147).
• To analyze the caloric curves and heat capacity to identify fine structures near the melting point.
• To understand the influence of cluster size and electronic shell structure on melting behavior and atomic diffusion.

Main Methods:

• Calculation of caloric curves for sodium clusters using molecular dynamics simulations.
• Analysis of heat capacity peaks to pinpoint transition temperatures.
• Observation and analysis of atomic diffusion and vacancy dynamics within the clusters.

Main Results:

• Both N=139 and N=147 sodium clusters displayed fine structures in their caloric curves near the solid-to-liquid transition.
• Surface melting was not observed in either cluster size.
• The N=139 cluster showed surface vacancy diffusion and became completely liquid shortly above its melting point.
• The N=147 cluster, a closed-shell system, maintained a rigid inner core of 13 atoms significantly above the melting temperature of its outer layers.

Conclusions:

• The electronic shell structure significantly influences the melting behavior of sodium clusters.
• Closed-shell clusters like N=147 can exhibit enhanced core stability, leading to heterogeneous melting.
• The observed differences in melting dynamics between N=139 and N=147 provide insights into size- and shell-dependent phase transitions in atomic systems.