Dynamic processes in the superionic conductor LaF3 at high temperatures as studied by spin-lattice relaxation dispersion

By nuclear magnetic resonance spin-lattice relaxation dispersion in LaF3, measured in the frequency range from 60 kHz to 284 MHz, dynamic processes with correlation times in the range from 10-5 to 10-10 s are studied. This allows us to trace fluorine dynamics in the temperature range from 303 to 1400 K. Two motional modes, a fast one and a slow one, are identified. Both motional processes are found not to be of Bloembergen-Purcell-Pound type but obey a log-Gaussian distribution of correlation times, thus reflecting the potential energy landscape in the superionic state. Below 1000 K, the activation energy of fluorine diffusion is 0.36 eV for the fast ions, and 0.57 eV for the slow ones. At higher temperatures, the activation energies change drastically. Above 20 MHz, where most relaxation studies have been performed so far, an additional contribution to relaxation, which is probably induced by paramagnetic centres, is found to be dominant. (author)