Thermal evolution of cobalt nanocrystals embedded in silica

The structural evolution of cobalt nanoclusters synthesized in silica glass by ion implantation has been investigated upon thermal annealing. The samples were characterized by in-situ grazing incidence X-ray diffraction, exploiting a synchrotron radiation beam and following their evolution during thermal treatments in vacuo up to T=800 degrees C. Before heating, the system is composed of hcp Co nanocrystals; we have not detected the transition from hcp to fcc structure that in the bulk phase occurs around 420 degrees C; nevertheless, the differences in the diffraction pattern recorded at T=800 degrees C with respect to the corresponding one at room temperature suggest the presence of a second crystalline phase.