Size effect on the crystal phase of cobalt fine particles

We have synthesized Co fine particles with the average diameter (D) of less than 500 Angstrom by sputtering Co in a somewhat high inert-gas pressure. It has been found that there is a close relationship between the particle size and the crystal phase; that is, pure fee (beta) phase for D less than or equal to 200 Angstrom, a mixture of hcp (alpha) and beta phases for D similar to 300 Angstrom, and alpha phase with inclusion of a very small amount of beta phase for D greater than or equal to 400 Angstrom. Precise structural characterizations have revealed that the beta particles are multiply twinned icosahedrons and the alpha particles are perfect single crystals with external shape of a Wulff polyhedron. In order to explain the size effect on the crystal phase of Co fine particles, we have performed theoretical calculations for total free energies of an alpha single crystal, a beta single crystal, and a multiply twinned beta icosahedron. The present calculations well explain the size dependence of the crystal phase of the Co fine particles, and have revealed that the stabilization of beta phase, confirmed by previous studies, is the intrinsic effect caused by the small dimensionality of fine particles. Moreover, the phase transformations that occurred in annealing experiments can also be explained by the theory.