The structure of vitreous silica

Abstract

A new study of the structure of vitreous silica has been made under greatly improved conditions. Using Rh Kα radiation with the method of fluorescence excitation, reliable intensity values were measured to 4π sinθ/λ = 20. The interpretation was in terms of pair functions, thereby eliminating the approximations in earlier work. Each silicon is tetrahedrally surrounded by 4 oxygen atoms, with a Si–O distance which is closely 1.62 Å. Each oxygen atom is bonded to 2 silicon atoms. The Si–O–Si bond angle α shows a distribution V(α) extending all the way from 120° to 180°, with a maximum at α = 144°. This wide variation in α is an important distinction between the vitreous and the crystalline forms of silica, and it furnishes an important criterion for any proposed model of vitreous silica. Good agreement with the measured pair function distribution curve was obtained by assuming a random orientation about the Si–O bond directions. The interpretation leads to the familiar random network model, but with the new results the model is more precise.

Ultraviolet-light-induced processes in germanium-doped silica

“…The model's explanation of the steps in the curve is that each step represents the contribution from one layer of neighbors to a germanium atom. The distance to these layers is determined from x-ray spectroscopy, 46 so the good agreement concerning the relative time for these steps is not due to the combined effect of the many free parameters but is an intrinsic property of the model. The number of steps is given by the x-ray pair function distribution curves and the relative timing and step height are predominantly determined by the r Ϫ8 dependence.…”

“…X-ray data fix the value of r 1 to 1.61 Å. 45,46 Finally, I estimate the saturation intensity I sat to be around 10 MW/cm 2 and the decay rate ⌫ to be around 10 9 s Ϫ1 . With these basic values fixed it is possible to obtain the values of all the free parameters.…”

“…The radial density function (r) is determined from experimental x-ray diffraction data for silica 45,46 starting at a silicon atom, e.g., the sum of the Si-Si and Si-O pair function distribution curves. This implies that the defect generation is considered to be equally likely at all atoms in the glass.…”

Structure of GeO2–P2O5glasses studied by x-ray and neutron diffraction

“…The parameters of the Gaussian functions are the coordination numbers, N ij , the mean distances, r ij , and the peak widths (full widths at half maximum), ∆r ij , for the pairs of atomic species i and j. Gaussian fitting is performed to the T N (r) and T X (r) data simultaneously. The effects of termination of the FT integral at Q max are taken into account by convolution of the Gaussian peaks with functions P ij (r) [25,26]. This procedure simulates the Q-window and damping used in equation (1).…”

On the nature of pressure‐induced coordination changes in silicate melts and glasses

“…It is generally accepted that silicon is tetrahedrally coordinated by oxygen ions in silicate melts and glasses at 1 atm total pressure and that adjacent silicate tetrahedra form polymers by comer-sharing [e.g., Mozzi and Warren, 1969;Taylor and Brown, 1979;Okuno and Marumo, 1982]. By analogy with the structures of crystalline silicates at pressures of several tens of GPa and based on shock wave studies of the Hugoniot states of molten silicates [Rigden et al, 1984[Rigden et al, , 1987, and in preparation], it is likely that silicon is more highly coordinated by oxygens at high pressures.…”