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 attemp to extract some general trends about the structural
and thermal behaviors of heterogeneous alkali clusters.
