Natrium-23 und Lithium-7 NMR-Untersuchungen an Silicat- und Boratgläsern / 23Na and 7Li NMR Studies of Silicate and Borate Glasses

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Abstract
The 23Na and the 7Li NMR spectra from several binary- and ternary alkali silicate and alkali borate glasses have been examined as a function of composition and temperature. At temperatures well below room temperature both the magnetic dipolar interactions between various nuclei and the electric quadrupolar interactions with the surroundings remain more or less the same in all the glasses. Upon increasing the temperature the width of the central transition of the 7Li absorption decreases, the 23Na dispersion changes from a quadrupolar to a more dipolar like shape, and important spin lattice relaxation is produced. Substitution of other alkali ions for lithium and sodium, respectively, affect the dynamic rather than the static behaviour of the glasses. The results are discussed with the assumption that the main contribution to the NMR signal is from nuclei belonging to alkali-rich regions which look similar in silicate and borate glasses, and for different alkali ions. The phenomena of motional narrowing, temperature dependent relaxation and drastic change of dispersion spectra are attributed to the diffusion of the alkali cations. From the relaxation data activation energies between 14 and 22 kcal/Mol have been obtained. Particular attention has been paid to the mixed alkali effect.

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