Electron trapping and hydrogen atoms in oxide glasses

Abstract

Trapped hydrogen atoms generated in 3 MeV β-radiolysis of B2O3:OH glass below 140 K were studied using electron paramagnetic resonance (EPR). Two types of trapped H atoms were present in this glass; one was an interstitial atom located in a void between several BOB fragments, another was the atom trapped in a cage between two B3O6 (boroxol) rings connected by hydrogen bonds. The geometry of the trapping site was determined using electron spin echo envelope modulation (ESEEM) spectroscopy. Time-resolved pulsed EPR was used to observe mobile H atoms at 300–500 K. The lifetimes (10–100 μs) of the H atoms were controlled by ∼1018 cm−3 of metastable spin centers. The H atoms migrated with diffusion constant of 1.5×107 cm2/s (activation energy of 0.13–0.16 eV), mean residence time at the site of 4–5 ns, and mean jump length of 0.56 nm (at 300 K). This site-to-site migration causes rapid spin relaxation due to modulation of magnetic interactions, such as dipole–dipole interaction of the unpaired electron of the H atom with B10 and B11 nuclei. Though there was no observed H/D kinetic isotope effect on the decay/diffusion of the hydrogen atoms, there was a significant isotope effect on their radiolytic yield (α≈1.5–1.6). This effect is comparable to the one observed in SiO2:OH and aqueous acid glasses. This similarity suggests that in the room-temperature “wet” SiO2 and B2O3 glasses, mobile H atoms are generated via electron trapping at the proton(s) associated with threefold coordinated oxygen (–OH2+ and/or >OH+ centers). Semiempirical MNDO simulations were used to estimate energetics of such electron trapping reactions.