The F mobility at various structural positions in monocrystals with the tysonite structure was analyzed by using 19F nuclear magnetic resonance line-shape analysis. The method was sensitive to ionic exchange, with correlation times of 10-6 to 10-3s. At 240 to 400K, motion was restricted mainly to F- ions in F-layers perpendicular to the main symmetry axis (the Fi sub-lattice), while F- ions in the La plane (F2,3) remained immobile. No significant anisotropy of Fi- ionic diffusion, within the layers and along the c-axis, was found. Both were approximately equal to 6 x 10-14m2/s at 400K. It was clear, from the nuclear magnetic resonance spectra, that the Fi mobility was strongly heterogeneous. The motional disorder could be well-described by a broad distribution of correlation times, G(τ), which had a shape that was close to log-Gaussian and which reflected the potential energy landscape in the superionic state. The variation in the center position, and in the width of G(τ) with temperature, differed from Arrhenius-law behavior. Therefore, the ionic mobility at the microscopic scale could not be considered to be a process which was only thermally activated. The use of molecular dynamics techniques showed that the presence of vacancies could lead to pronounced changes in the potential energies, and supported the concept that there existed a distribution of activation energies.

The Distribution of Motional Correlation Times in Superionic Conductors - 19F Nuclear Magnetic Resonance of Tysonite-Like LaF3. A.F.Privalov, A.Cenian, F.Fujara, H.Gabriel, I.V.Murin, H.M.Vieth: Journal of Physics, 1997, 9[43], 9275-87