NMR and NQR studies of boron in vitreous and crystalline berates

Nuclear magnetic resonance, (NMR) and nuclear quadrupole resonance (NQR) ha ve both been used to study boron in crystalline berates and glasses contain ing boron oxide (B2O3). Both boron nuclear isotopes, B-10 and B-11, have ma gnetic moments and electrical quadrupole moments. The interactions of the l atter perturb the NMR Zeeman energy levels and produce broadening and struc ture in the NMR spectra. Computer analysis of that structure yields quadrup olar parameters that depend very sensitively on the boron coordination, che mical bonding, and the identity of structural groupings containing the boro n atoms. NMR spectra for three- and four-coordinated borons are easily dist inguished from each other and the fractions of boron in each coordination c an be obtained. Similarly, three-coordinated borons bonded to different num bers of non-bridging oxygens are distinguishably and can be quantified. All boron sites are 'fingerprinted' by the quadrupolar parameters that can be used to check the validity of values calculated by: quantum mechanical. pro cedures. NQR spectra occur at low frequencies (below 1.4 MHz) which poses e xperimental problems, but they can yield data that are much more accurate t han the NMR-derived results, and resolve responses that overlap or conceal each other in the NMR spectra. Identification in glasses of the type and am ount of structural groupings found in crystalline berates provides guidance for compositional variations or heat treatments that produce desired macro scopic properties of the glasses. (C) 1999 Elsevier Science S.A. All rights reserved.