23Na nuclear magnetic resonance spin echo decay spectroscopy of sodium silicate glasses and crystalline model compounds

Abstract

The possibility of using Hahn spin echo decay spectroscopy for measuring 23Na- 23Na dipole-dipole couplings in crystalline solids and glasses is investigated. Although the presence of nuclear electric quadrupolar splittings complicates the situation compared to the case of dipole-dipole couplings among spin- 1 2 nuclei, experimentally measured second moments lie within approximately 10–20% of the calculated values, if the following conditions are fulfilled: (1) selective excitation of the central 1 2 → − 1 2 transition, and (2) restriction of the analysis to short evolution times (2 t 1 ≤ 200 μs). At longer evolution times, partial suppression of the spin-exchange term in the dipolar Hamiltonian due to magnetic inequivalencies between the interacting nuclei produces substantial differences between experimental and calculated spin echo decays, the extent of which depends on the compound considered. These results signify the potential utility of 23Na spin echo nuclear magnetic resonance for testing the distribution of Na-Na distances in structurally disordered solids and glasses. The method is shown to be applicable to sodium silicate glasses, where it reproduces anticipated compositional trends in 23Na- 23Na couplings. Details and inherent limitations of this methodology are discussed.