NMR and NQR studies of boron in vitreous and crystalline borates

Autor: Philip J. Bray
Rok vydání: 1999
Předmět:
Zdroj: Inorganica Chimica Acta. 289:158-173
ISSN: 0020-1693
DOI: 10.1016/s0020-1693(99)00066-3
Popis: Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) have both been used to study boron in crystalline borates and glasses containing boron oxide (B2O3). Both boron nuclear isotopes, 10B and 11B, have magnetic moments and electrical quadrupole moments. The interactions of the latter perturb the NMR Zeeman energy levels and produce broadening and structure in the NMR spectra. Computer analysis of that structure yields quadrupolar parameters that depend very sensitively on the boron coordination, chemical bonding, and the identity of structural groupings containing the boron atoms. NMR spectra for three- and four-coordinated borons are easily distinguished from each other and the fractions of boron in each coordination can be obtained. Similarly, three-coordinated borons bonded to different numbers of non-bridging oxygens are distinguishable 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 procedures. NQR spectra occur at low frequencies (below 1.4 MHz) which poses experimental problems, but they can yield data that are much more accurate than the NMR-derived results, and resolve responses that overlap or conceal each other in the NMR spectra. Identification in glasses of the type and amount of structural groupings found in crystalline borates provides guidance for compositional variations or heat treatments that produce desired macroscopic properties of the glasses.
Databáze: OpenAIRE
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