Allotropic phase transformations in HCP, FCC and BCC metastable structures in Co-nanoparticles

The effect of particle size on stability of crystal structure of pure cobalt metal is studied. The common HCP allotrope of it is stable in bulk. It assumes an FCC or a BCC structure on reducing its size below a critical value Rc at a nanometer scale. Separated Co-particles coreduced (using a reducing agent) from highly dispersed Co2+-cations in a liquid or a solid medium have a strict control of size at D=2–5 nm for the BCC structure and D=10–20 nm for the FCC structure. Both of them are substantially stable at least within the temperature range of 700°C and do not transform to the HCP structure as long as their size is controlled within these regions. The present crystal structure modification in a particle occurs in a manner that it has a minimal value of its total surface energy Ω=Aσ, with A its total surface area and σ the surface-energy density. It yields a stable equilibrium configuration of atoms with its minimal internal energy.