Atomic structure and segregation in alkali-metal heteroclusters

The ground-state atomic and electronic distributions in ${\mathrm{Na}}_{\mathit{m}}$ ${\mathrm{Cs}}_{\mathit{n}}$ clusters with composition m=n and m=2n have been calculated by minimizing the total cluster energy using the density-functional formalism. The approximation is made by replacing the total external potential of the ions by its spherical average around the cluster center during the iterative process of solving the Kohn-Sham equations for each geometry tested. In the size range studied here (up to 90 atoms per cluster), the cluster is composed of well-separated homoatomic Na and Cs shells, the external one always being a Cs shell. We have also found that the cohesive energy goes rapidly to the bulk limit. An analysis of the geometries shows strong cluster reconstruction with increasing size. By comparing the geometry of pure ${\mathrm{Na}}_{\mathit{n}}$ with that of the ${\mathrm{Na}}_{\mathit{n}}$ core in ${\mathrm{Na}}_{\mathit{n}}$ ${\mathrm{Cs}}_{\mathit{n}}$ for clusters formed by only an inner Na layer and an outer Cs layer, we have observed that the ${\mathrm{Na}}_{\mathit{n}}$ core adopts a geometry different in most cases from that of the free ${\mathrm{Na}}_{\mathit{n}}$ cluster, and such that the number of faces of the polyhedron formed by the ${\mathrm{Na}}_{\mathit{n}}$ core is as close as possible to the number of external Cs atoms, in order to accomodate these Cs atoms on top of the faces of the polyhedron.