SiO2 xerogel and aerogel treated up to 900oC exhibit amorphous behavior. A narrowing of
the XRD peak accompanied by an increase in its intensity with increasing in the temperature
of the preparation indicate an increase in the structural organization of the samples (Figure
3a and 3b). Characteristic reflections of crystobalite and tridimite appear at 1100oC for both
the samples. The intensity of the xerogel peaks are larger than the aerogel ones.
The xerogel nanocomposites exhibit amorphous behavior up to 300 oC (according the X-ray
difractometer resolution). CoFe2O4 crystalline particles with cubic spinel structure are detected
by XRD inside the amorphous silica matrix above this temperature (Figure 4a). During
the formation of the CoFe2O4 nanocrystals, no traces of intermediate products are found
even at temperatures as high as 11000C, indicating that the ferrite particles were formed
without binding to the matrix. The magnetic nanoparticles also avoided the formation of either
crystobalite or tridimite phases.