Glass structure

Abstract

Nuclear magnetic resonance (NMR) studies of the structure of glasses have been carried out for more than 25 years with particular attention having been focused on borate glasses. NMR spectra for 11B have been used to determine the fractions N 4 and N 3 of boron atoms in BO 4 and BO 3 configurations, respectively. 11B spectra also yield the fractions N 3S and N 3A of BO 3 configurations which are symmetric (0 or 3 non-bridging oxygens) and asymmetric (1 or 2 non-bridging oxygens). The interaction between the nuclear electrical quadrupole moment and the electric field gradient (which arises principally from the bonding electron) gives rise to structure in the NMR spectra which can be analyzed to yield N 4, N 3, etc. More recently, 10B NMR spectra have been obtained which are much more sensitive than 11B spectra to changes in chemical bonding and atomic arrangements. These spectra permit identification in the glasses of structural groupings [e.g. (B 4O 7) −2, (B 3O 6) −3, (B 8O 13) −2] which are present in the crystalline compounds of the oxide glass-forming system, and also yield quantitative measures of the concentration of each grouping in a glass. These data permit construction of detailed structural models for glasses in systems such as Na 2OB 2O 2, Na 2OB 2O 3SiO 2, and PbOB 2O 3SiO 2. Quadrupole effects in NMR spectra of 17O for glasses enriched in this isotope have made possible identification of boroxol units in vitreous B 2O 3 and determinations of SiOSi bond angles in vitreous SiO 2. Chemical shift effects in the NMR spectra of 207Pb have shown the presence in lead borate, silicate, and borosilicate glasses of the lead-oxygen bonding configurations present in the crystalline compounds of the system. Anisotropic chemical shifts in 29Si spectra for K 2OSiO glasses reveal the same silicon-oxygen bonding configurations in the glasses and crystalline compounds of the same composition, and major differences in the spectra for the various crystalline compounds.