Molecular dynamics simulations of the meltinglike transition in Li13Na42 and Na13Cs42 clusters

Equilibrium geometries and the meltinglike transition of Na13Cs42 and Li13Na42 are studied by means of orbital-free density-functional-theory molecular dynamics simulations. A polyicosahedral structure is found to be energetically favored for Na13Cs42, with a core shell formed by Na atoms and complete segregation of Cs atoms to the cluster surface. Li13Na42 adopts an amorphouslike structure, albeit with significant local polyicosahedral order, with the Na atoms preferentially occupying surface sites but with partial mixing of Li and Na species at the cluster core. Analysis of the thermal properties reveals that premelting effects are more important for heterogeneous than for homogeneous alkali clusters. The nature of these premelting effects is discussed in detail. For Na13Cs42, they involve isomerizations without significant atom diffusion; for Li13Na42, they also include partial melting of the surface formed by Na atoms. The mixing of Li and Na species is significantly enhanced above the melting temperature, while surface segregation of Cs in Na13Cs42 is maintained in the liquid state. From the study of these two clusters, we attempt to extract some general trends about the structural and thermal behaviors of heterogeneous alkali clusters.