LI-7 NMR-SPECTRA, NUCLEAR-RELAXATION, AND LITHIUM ION MOTION IN ALKALI SILICATE, BORATE, AND PHOSPHATE-GLASSES

Investigations of the 7Li nuclear magnetic relaxation time T1 and of the broad line spectra over a wide temperature range are reported for binary and ternary alkali silicate, alkali borate, and alkali phosphate glasses. Some measurements in low effective fields (T1D, T1ρ{variant}) have also been made. Second-moment data yield information concerning the distribution of the alkali ions in the glass. Motional narrowing is governed by a mixture of dipolar and quadrupolar effects. There are two relaxation mechanisms, one due to diffusion, which is operative between approximately 250 K and the transformation temperature, and the other one operative below. The high-temperature Zeeman relaxation is dominated by the time-dependent coupling of the lattice to the quadrupole moment, the dipole relaxation by the dipole-dipole interaction. The temperature dependences can be explained only by a distribution which incorporates decreasing values of correlation times. Typical quantities such as the activation energy for ion diffusion and the width parameter are reported and discussed in terms of the glass composition. Anomalous relaxation at low temperatures has been found also to be related to the distribution of correlation times, and it is suggested that the relaxation be described by two-level tunneling modes in a double minimum potential. T1 T1D ratios of the order of 100 have indicated the existence of slow motions in this region. © 1979.