Si - O - Si strained bond and paramagnetic defect centers induced by mechanical fracturing in amorphous SiO₂

Mechanical stress is applied to high-purity amorphous SiO₂ samples by means of fracturing. The electron-spin-resonance spectra suggest that the formation of a majority of paramagnetic defects is from cleavage of Si - O - Si bridges in the glass network, but there are some sample-to-sample variances in fracture-induced paramagnetic defects, suggesting cleavage of differing chemical bonding states in the samples. Nonstoichiometric bonds, ≡Si - Si≡ and ≡Si - O - O - Si≡, are assumed to be one reason for the sample dependency. Formation of Si - O - Si strained bonds from mechanical fracturing is confirmed from sequential γ-ray irradiation and heat annealing experiments. The Si - O - Si strained bond is approximately annealed at about 300°C. By comparing the fracture-induced defects for glass preforms and optical fibers, the change in chemical bonding state can be analyzed. Analysis of mechanical-fracture-induced defects is a strong technique for elucidation of the chemical bonding state of silica glass.