Electronic structure and stability of polycrystalline cobalt clusters -: art. no. 045420

We study the electronic structure and stability of Co((n+m)) clusters with mixed (bcc(n)-fcc(m)) structures [57 less than or equal to (n + in) less than or equal to 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 bee-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 fee growth sequence, a fact that reveals the important role played by the polymer solution used in the stabilization of structural phases in small particles.