Electron trapping and hydrogen atoms in oxide glasses

Abstract

Absence of recombination of neighboring H atoms in highly purified solid parahydrogen: Electron spin resonance, electron-nuclear double resonance, and electron spin echo studies Transient nucleation in oxide glasses: The effect of interface dynamics and subcritical cluster population Trapped hydrogen atoms generated in 3 MeV ␤-radiolysis of B 2 O 3 :OH glass below 140 K were studied using electron paramagnetic resonance ͑EPR͒. 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 B 3 O 6 ͑boroxol͒ rings connected by hydrogen bonds. The geometry of the trapping site was determined using electron spin echo envelope modulation ͑ESEEM͒ spectroscopy. Time-resolved pulsed EPR was used to observe mobile H atoms at 300-500 K. The lifetimes ͑10-100 s͒ of the H atoms were controlled by ϳ10 18 cm Ϫ3 of metastable spin centers. The H atoms migrated with diffusion constant of 1.5ϫ10 7 cm 2 /s ͑activation energy of 0.13-0.16 eV͒, mean residence time at the site of 4-5 ns, and mean jump length of 0.56 nm ͑at 300 K͒. 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 10 B and 11 B 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 (␣Ϸ1.5-1.6). This effect is comparable to the one observed in SiO 2 :OH and aqueous acid glasses. This similarity suggests that in the room-temperature ''wet'' SiO 2 and B 2 O 3 glasses, mobile H atoms are generated via electron trapping at the proton͑s͒ associated with threefold coordinated oxygen ͑-OH 2 ϩ and/or ϾOH ϩ centers͒. Semiempirical MNDO simulations were used to estimate energetics of such electron trapping reactions. ͓S0021-9606͑99͒52335-0͔

Hydrogen-atom-mediated electrochemistry

“…Thus, the observed voltammetric results can be understood by a scenario in which an external electric field causes protons to migrate in the oxide layer 24 and the protons are electrochemically reduced to produce H atoms 9,32 in a negative potential regime. Shkrob et al 10 suggested that electrons react with network-bound protons at 180-530 K to produce H atoms.…”

“…Thermal SiO 2 is a dielectric material used for conventional metal/insulator/metal capacitors 6 , and SiO 2 films are widely used as gate insulators of metal/oxide/semiconductor (MOS) devices 7 , in which the field-induced drift of protons to the Si/SiO 2 interface and their interactions with electrons can create hydrogen (H) atoms at the interface 8,9 . Shkrob et al 10 demonstrated that protons were reduced to generate mobile H atoms in B 2 O 3 glasses at room temperature. Energy calculations revealed that H atoms in SiO 2 are isolated by themselves without a strong interaction with oxygen (O 2 ) or Si atoms.…”

Radiation Effects on Transport and Bubble Formation in silicate Glasses

“…There is no clear distinction between different variants of the hole centers in alkali borate glasses. The observed difference in the spin parameters is due to variations of the immediate environment of the BO 3 -unit. The dangling bond center is an electron trapped on over coordinated oxygen (the "O 3 + " defect).…”

“…All variants of oxygen hole centers are holes trapped on the nonbridging oxygen in a BO 3 -unit. Alkali cations and protons are excluded from this center; the hole is trapped on under coordinated oxygen (the "O 1 -" defect).…”

“…If the central oxygen is coordinated with borons, the neutral center is formed. If it is coordinated with an alkali cation or a compensated BO 3 -unit, a charged center is formed. The "O 3 + " defect is a deeper trap for electrons than an isolated network-bound alkali cation.…”