Thermal history dependence of the crystal structure of Co fine particles

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hcp and fcc Co fine nanoparticles with different size have been synthesized by low-temperature laser pyrolysis and by reduction at different temperatures from $200{{0.2em}{0ex}}^∘{}C{{0.3em}{0ex}}to{{0.3em}{0ex}}550{{0.2em}{0ex}}^∘{}C$ of cobalt-oxide nanoparticles prepared by the same method. The influence of thermal treatment on the crystal structure of Co particles was examined. It was found that below $420{{0.2em}{0ex}}^∘{}C$, only hcp Co phase particles were obtained by laser pyrolysis or by reduction. Medium temperature (around $420{{0.2em}{0ex}}^∘{}C$) resulted in a mixture of hcp and fcc phases. Only when high temperatures, above $500{{0.2em}{0ex}}^∘{}C$, were involved during the synthesis or annealing of Co particles, did the fcc particles form and maintain their structure to ambient temperature, without structural change of fcc-hcp. In view of the size effect on the phase transformation kinetics, the stabilization of fcc Co particles was explained.