Atomic structure and correlation in vitreous silica by X-ray and neutron diffraction

Abstract

X-ray and neutron diffraction patterns from vitreous silica (SiO 2) have been measured, and each of the patterns was analyzed to produce an atomic radial distribution function. The two independently obtained distributions differ considerably due to the different relative magnitudes of the scattering amplitudes of the two atom-types for each of the forms of radiation. The experimental results were combined in a ratio calculation which clearly establishes that each of the first three peaks in the atomic distribution represents a single type of atom pairing; Si-O pairs with a 1.6 Å inter-atomic separation, O-O pairs at 2.6 Å and Si-Si pairs at 3.1–3.2 Å. The distribution function peaks for larger separations are shown to represent mixtures of all three types of pairing. This structure is consistent with the random-network theory first proposed by Zachariasen. The validity of the two-pattern ratio method for determining correlation in binaries, or for testing theoretical models, is established.