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

The effect of particle size on stability of crystal structure of pure cobal t metal is studied. The common HCP allotrope of it is stable in bulk. It as sumes an FCC or a BCC structure on reducing its size below a critical value R-c at a nanometer scale. Separated Go-particles coreduced (using a reduci ng 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 l east within the temperature range of 700 degreesC and do not transform to t he 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 Omega = A sigma, w ith A its total surface area and sigma the surface-energy density. It yield s a stable equilibrium configuration of atoms with its minimal internal ene rgy. (C) 2001 Elsevier Science. B.V. All rights reserved.