Paramagnetic centers in SiO2 glass

Electron Paramagnetic Resonance (EPR) of ten samples of SiO2 glass was stud ied 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. T he 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 sin gle 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 s uggested that these transition metal ions are some part of the sample segre gated in the surface of some grain.