The Eδ′ and triplet-state centers in x-irradiated high-purity amorphous SiO2

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Abstract
High-purity silicas synthesized by the chemical-vapor-deposited soot remelting method were studied by electron-spin-resonance techniques after being irradiated by x rays at 77 K or higher temperatures. The spectra of the Eδ′ center including its 29Si hyperfine splitting, and the triplet-state center, were measured using two different detection modes. The effects of x-ray dose, thermal annealing, hydrogen treatment, and impurities were examined; the Eδ′ and the triplet-state centers have a similar dependence on all these parameters, indicating that they share a common precursor. These centers are found only in low OH, oxygen-deficient samples. There appears to be no correlation with chlorine impurities. The measured intensity of the Eδ′ center’s 29Si hyperfine signal indicates that approximately four Si atoms are involved. A model for this center and the triplet-state center is proposed.