Fluctuation electron microscopy study of crystal nucleation in TiO ₂ –SiO ₂ glass with heat treatment

We have used fluctuation electron microscopy (FEM) to investigate the nucleation stage of TiO 2 crystal formation in binary TiO 2 –SiO 2 glasses with heat treatment. It was found that spatial fluctuations of electron scattering in the glass with 13 wt% TiO 2 increases with heat treatment above 800 °C but before any crystals precipitate, i.e. before crystals are detectable by electron diffraction. We have attributed this to TiO 2 clustering and increasing medium-range order due to gradual ordering of TiO 2 phase. Moreover, we have found that FEM is sensitive to structural changes at temperatures as low as 400 °C but the nature of the changes yet to be determined. This demonstrates that FEM can be sensitive to structural changes in oxide glasses occurring during thermal treatment but preceding detection of the first crystals.