NMR Study of Structure and Bonding in Glasses

Abstract

Nuclear magnetic resonance (NMR) techniques have been employed to study the bonding between atoms, and arrangements of atoms, in glasses. Oxygen bonding in SiO₂ and B₂O₃ has been investigated using the oxygen-17 isotope. The O¹⁷ NMR spectrum for vitreous SiO₂ (v-SiO₂) exhibits well-defined structure and indicates significant local order in the material. Matching of experimental and computer-simulated spectra permits extraction of the O^l7 quadrupole coupling parameters which are analyzed to obtain charge densities in the electronic orbitals and the Si-O-Si bond angles and distributions. Comparisons can be made with values obtained from the O¹⁷ NMR spectrum of a-cristobalite. Both boron and oxygen have been studied in v-B₂O₃. The B¹⁰, B¹¹, and O¹⁷ NMR spectra yield quadrupole parameters, bond angles, and charge densities. Two distinct types of oxygen site are clearly revealed which are consistent with the boroxol ring oxygens and the ring-connecting oxygens. B¹⁰ studies of Na₂O-B₂O₃ and Li₂O-B₂O₃ glasses yield spectra which are beautifully simulated by adding the spectra from the crystalline compounds in the respective systems. The relative amount of the structural grouping (i.e., boroxol, diborate, metaborate) from each compound can then be determined in each glass and followed as a function of composition. B¹¹ studies in the technologically important sodium borosilicate system have permitted construction of structural models for the glasses in this system. The models yield the amount of each type of borate grouping present in each glass.