Vitreous silica atomic structure

The tetrahedral network can be considered the idealized structure of vitreous silica. Disorder is present but the basic bonding scheme is still intact. An additional level of disorder occurs because the atomic arrangement can deviate from the fully bonded, stoichiometric form through the introduction of intrinsic (structural) defects and impurities. These perturbations in the structure have significant effects on many of the physical properties. A key concern is whether any of these defects breaks the Si—O bonds that hold the tetrahedral network together. Fracturing these links produces a less viscous structure which can respond more readily to thermal and mechanical changes. 

Chemical Properties. Stoichiometric vitreous silica contains two atoms of oxygen for every one of silicon, but it is extremely doubtful if such a material really exists. In general, small amounts of impurities derived from the starting materials are present and various structural defects can be introduced, depending on the forming conditions. Water is incorporated into the glass structure as hydroxyls. 

Paramagnetic centers with an unpaired electron localized on the three-coordinated silicon atom are formed in vitreous silica under exposure to various factors [10,104,105]. In the presence of nitrogen atoms in glass, a portion of these atoms can occur as the (=Si-0-)2Si (-N<) structure. However, to the best of the author s knowledge, the occurrence of these centers in... 

Molecular hydrogen is one of the few compounds that can be introduced into the bulk of vitreous silica. Chemical processes with the participation of molecular hydrogen are used for modifying the structure of this widely used material. The defects of vitreous silica play an important role in these processes. The exposure of =Si-N -H radicals to an atmosphere of H2 at room temperature is accompanied by the chemisorptions of the gas in an amount that is comparable with the number of radicals and by the disappearance of the EPR spectrum of these radicals (radiospectroscopic measurements were performed at 77 K (see Section 12.1)). When diamagnetic centers of the silylene type ((=Si-0)2Si ) also occurred on the sample surface in addition to the PC =Si-N -H, the decay of radicals in an atmosphere of H2 was accompanied by the quantitative formation of new PCs >Si -H. The appearance of the > Si -H radicals in the system suggests that H atoms... 

We consider first the simulation of the atomic structure of vitreous silica because the majority of the simulations of amorphous oxides were done for this material. Some of these have simulated the formation of the vitreous silica surface in a very detailed fashion. Furthermore, the methods developed for the simulation of vitreous silica and its surface may be used with some modifications for other amorphous oxides. Subsequently, we consider less detailed methods of simulation of amorphous oxide surfaces which are not limited to Si02 but can be applied to various oxides. Finally the least detailed but the most general model - the Bernal surface (BS) - represents the atomic arrangement at the surface of any amorphous oxide (most important for physical adsorption) by the dense random packing of hard spheres. 

Three related structural parameters for characterizing the atomic-scale structure of vitreous silica are the Si-O-Si bond angle between adjacent tetrahedra, the rotational angle between adjacent tetrahedra, and the rings of oxygens, as illustrated in Fig. 7 [5], Each of these parameters has a constant value or set of values in crystalline silica,... 

Vessal and Wright (manuscript in preparation) have started from a vitreous silica structure generated by MD (Vessal et al., 1993) with 648 atoms in a cubic box of 21.397 A on each side. The MD structure used is the closest MD model to the experiment and exhibits an Rx factor of 9.1 % calculated between 1 and 8 A (Wright, 1993). They have attempted to refine the MD structure using RMC. This has resulted in a structure that is very close to neutron diffraction results Rx of 1.6% calculated between 1 and 8 A - see Fig. 12.26). This structure, however, remains unsatisfactory because it yields an average coordination number of 3.74 for silicons. Other applications of RMC methods to silicate systems are given in Chapter 6. 

Recent dramatic advances in computational techniques and computer power have enabled us to simulate crystalline structures from first-principles by means of the electronic structure calculation of the whole system within the density functional theory. Even liquid and vitreous silica have come to be studied by the ab initio MD method or so-called Car-Parrinello method [59]. Thus the application of the classical MD method is to be shifted to study of dynamics with a larger system size and longer simulation time. For example, the simulation of the oxygen diffusivity mentioned in the previous section needs accumulation of positions of five hundred atoms over 120 ps at each pressure, for which the ab initio MD is too inefficient. On the other hand, a local structural deformation relevant for the diffusion could be simulated with a smaller cell and a shorter time scale. It is obviously fruitful to make proper use ofthese approaches, i.e. the classical MD supported by first-principles cluster calculations and the ab initio MD, in each problem of materials science. 

The structure of monosilicic acid is assumed to involve silicon coordinated with four oxygen atoms as in amorphous vitreous silica and in crystalline quartz. Although there are rare minerals such as the stishovite form of SiOj (21) or thau-masite (22), in which silicon is coordinated with six o.xygen atoms, silicon in most oxides and silicates is surrounded by only four oxygen atoms. If the monomer had the structure HjSKOH), one would expect it to be a strong acid like the analogous HaSiF, but in fact it is a very weak acid. 

Example 1 Determination of coordination numbers. If one considers the spectra of vitreous silica and its different crystalline allotropic forms (quartz, cristobalite, etc.), one notices the presence of an infrared absorption band centered at 9.1 /rm in all spectra. As the structure of all those varieties is based on a tetrahedrally coordinated silicon atom, it is natural to assign this band to one of the fundamental vibrations of the Si04 tetrahedron. 

---