Motional Narrowing of Nuclear Magnetic-Resonance Lines in Manganese-Doped Lithium Fluoride

Nuclear magnetic-resonance experiments have been performed between 25 and 550°C in LiF powder doped with manganese. Motional narrowing of the ${\mathrm{Li}}^7$ resonance line occurred somewhat below 400°C. Resonance absorption lines were not observable in the temperature range 200-390°C, probably because an effective loss of paramagnetic relaxation centers increased the spin-lattice relaxation time. Electron spin-resonance experiments verified that the paramagnetic manganese does cluster upon heating above 200°C. A method of analysis is developed which eliminates errors due to constant linewidth contributions in the analysis of motional narrowing data in terms of diffusion activation energies. The activation energy for motion of Li ion vacancies in LiF is found to be 0.74 eV.