Structural behavior of laser-irradiated γ-FeO nanocrystals dispersed in porous silica matrix : γ-FeO to α-FeO phase transition and formation of ε-FeO.

The effects of laser irradiation on γ-FeO 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-FeO phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-FeO nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-FeO is an intermediate phase between γ-FeO and -FeO and a series of distinct Raman vibrational bands were identified with the ε-FeO phase. The structural transformation of γ-FeO into -FeO occurs either directly or via ε-FeO, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.