Determination of the Si–O–Si bond angle distribution in vitreous silica by magic angle spinning NMR

Abstract

A necessary condition for any proposed description of the structure of glass is that a model of the glass will reproduce the details of the pair correlation function which may be determined from elastic scattering experiments (X-ray, neutron or electron diffraction). From a knowledge of the structure, excitations (electronic or vibrational) may be deduced. The most extensively studied prototype glass is vitreous SiO₂. It has long been recognized that the SiO₂ tetrahedra are well preserved in the glass and that it is the interconnection of the tetrahedra which provides the clue to the lack of translational symmetry characteristic of the glassy state. The natural approach has been to build macroscopic models of the structure which satisfy the bond conductivity between atoms whilst preserving the regularity of the SiO₂ tetrahedra. When completed the models are scaled and tested against diffraction data and the macroscopic density. Here, magic angle spinning nuclear magnetic resonance (NMR) spectroscopy has been applied to glassy SiO₂. The observed line-shape allows the published distribution functions for the Si–O–Si bond angle, crucial in determining the structure of vitreous SiO₂, to be tested. All of the published distribution functions are inconsistent with our data and a new distribution is proposed.