H2OandO2molecules in amor

Abstract

Interstitial water and oxygen molecules are ubiquitous impurities and participate in various defect formation processes in thermally grown SiO 2 films and synthetic silica glass. Using results of first-principles calculations we report the types of defects ͑including different possible charge states͒ that H 2 O and O 2 molecules may form in bulk amorphous SiO 2 . We calculate their formation energies and, in the most interesting cases, the energy barriers in order to map out the most likely defect formation scenarios. In particular, we show that water molecules may form double silanol groups ͑Si-OH͒ as well as H 3 O ϩ and OH Ϫ ions at a low energy cost with a barrier of about 1.5 eV. The formation energies of other defects emanating from H 2 O interstitials are, however, too high to be thermally activated. We found that O 2 molecules may form ozonyl ͑Si-O-O-O-Si͒ linkages with an energy barrier of ϳ2.4 eV. An explanation for the oxygen isotope exchange observed in thin SiO 2 films near the Si-SiO 2 and SiO 2 -vacuum interfaces is suggested based on the energy barrier for ozonyl formation being commensurate with the O 2 diffusion barrier close to the Si/SiO 2 interface and the O 2 incorporation energy from vacuum. We also explain the different creation rates of EЈ centers in wet and dry oxides by studying the annihilation mechanism of neutral and charged oxygen vacancies.

Cleavage and Recovery of Molecular Water in Silica

“…This has not been revealed previously by ab initio cluster calculations. For glass B, the bound state is an intermediate local minimum (also seen in ref 20), whereas in glass A, the bound state is lower in energy than any free water configuration found in that glass.…”

“…Although studies of sol-gel reactions show that small rings are more reactive at the sample surface, 21,22 recent bulk calculations state that the formation energy of silanol group pairs is more favorable for larger (six-and eightmembered) rings. 20 Scission of a small ring may potentially reduce the strain associated with that small ring, but the rupture of a bond in a larger ring may allow a longer range relaxation involving folding or unfolding of the long network chain structure associated with the large ring. In addition, although breaking a bond allows the strain to relax, formation of the silanol groups also requires room to fit across the broken bond.…”

“…Recent work has addressed key issues in diffusion and reactions of water with respect to different ring sizes and defects in silica. 19,20 That work mentioned the existence of a range of activation barriers and a range of relative stability between the molecular water and silanol states. The intent of this study is to uncover the source of the ranges of the activation barrier and relative stability as well as to reveal the role of network relaxation in the reaction pathways.…”

From random inhomogeneities to periodic nanostructures induced in bulk silica by ultrashort laser

“…There were experimental evidences that the initial nonirradiated defects were present in fused silica and they were activated by femtosecond laser irradiation [7,27,31,[52][53][54][55][56][57], leading to the inhomogeneous localization of the multiphoton ionization [2,48]. The created nanoplasma affected the laser propagation pulse by pulse due to the presence of a feedback mechanism based on memory of previous nonlinear ionization [58].…”

Dynamics of Network Formers and Modifiers in Mixed Cation Silicate Glasses

“…Such hydroxyl groups are products of the hydrolysis of TEOS and likely remain in a sol-gel derived glass even after consolidation. During annealing in O-rich ambient the concentration of OH groups decreases due to the formation and outdiffusion of H 2 O [29]. Accordingly, diffusion of O via hydroxyl groups should be a transient process.…”