Observation of two different types of stable [FeO4−/Na+]0 centres in a silica glass

The EPR properties of the ubiquitous iron impurity in samples of a Vycor-type silica glass of different atmospheric sintering have been studied. As with the low-temperature observation of dynamic interchange between two configurations of the [FeO4−/Na+]0 centre, two similar stable iron centres of different configurations based on two different permanent locations of the Na+ ion have been found. The EPR resonances of the two centres partially overlap and exhibit g-values in the range of 4.20–4.28. Both hydrogen sintering and γ-irradiation of the glass were found to decrease the intensity of the signal of each centre by an equivalent extent. The phenomenon is rationalised by considering the possibility of chemical reduction of the iron centres in each of the two treatment processes.