Electronic structure and stability of polycrystalline cobalt clusters

We study the electronic structure and stability of ${\mathrm{Co}}_{\mathrm{}(n+m)\mathrm{}}$ clusters with mixed $({\mathrm{bcc}}_n-{\mathrm{fcc}}_m)$ structures $[57<~\mathrm{}(n+m)\mathrm{}<~177]$ by means of a self-consistent tight-binding Hamiltonian solved by molecular dynamics. This type of construction is motivated by recent high-resolution transmission electron microscopy images of Co nanoparticles [Phys. Rev. B 57, 2925 (1998)] from which the coexistence of a bcc-like crystalline arrangement with a distorted compact surface has been inferred. The minimization algorithm reveals that all the clusters undergo an inhomogeneous radial relaxation, which leads to a complex spatial distribution of the atoms within the structure. Interatomic distances which can be larger than those of bulk Co are obtained, together with a considerable surface reconstruction that favors the formation of nonplanar facets. In general, the total energy of the mixed particles increases as a function of the cluster size. However, these structures are almost in all cases less stable when compared with a pure fcc growth sequence, a fact that reveals the important role played by the polymer solution used in the stabilization of structural phases in small particles.