Paramagnetic centers in SiO2 glass

Electron Paramagnetic Resonance (EPR) of ten samples of SiO2 glass was studied at X-band and at room temperature. Spectra of the iron impurity in all these samples were observed even in those with Fe content below 0.06 ppm. The lines found with g-factors 6.5 and 3.7 were assigned to Fe2+ and with g = 2.1 to Fe3+. Sample prepared with addition of nickel at 50 ppm gave a single line at g = 2.219 due to Ni2+ ions in a cluster form. As the resonance of Fe3+ ions also might be interpreted as due to cluster formation, it is suggested that these transition metal ions are some part of the sample segregated in the surface of some grain.