NMR Studies of Diffusion in Glassy and Crystalline Lithium Borates

Abstract

The ⁷Li NMR spectra from several lithium borate glasses have been examined as a function of temperature and found to narrow abruptly with increasing temperature in the range of temperatures from 250° to 475°K. The observed narrowing is a manifestation of the phenomenon of motional narrowing, and is attributed to diffusion of the lithium ions in the glasses. Activation energies for this diffusion, as deduced from the temperature dependence of the ⁷Li NMR linewidths, range from 0.22 eV in a glass containing 28.6 molar % Li₂O to 0.12 eV in a 60 molar % Li₂O glass. The ⁷B NMR lines from these glasses are reduced somewhat with increasing temperature because of the removal of the lithium‐boron dipolar interaction with the onset of the lithium diffusion. The linewidths of the ⁷Li spectra from the crystalline lithium borates also decrease with increasing temperatures and the activation energies obtained from their narrowing rates range from 0.14 eV in the Li₂O·4B₂O₃ compound to 0.43 eV in the 3Li₂O·2B₂O₃; compound. It has been concluded that the ionic movement responsible for the narrowing of the resonance spectra is motion within limited regions, rather than the movement over long distances necessary for electrical conductivity.