The mobility of F at various structural positions in monocrystalline samples having the tysonite structure was determined by means of 19F nuclear magnetic resonance lineshape analysis. At 240 to 400K, motion was restricted mainly to the F- ions in the F layers perpendicular to the main symmetry axis (F1 sub-lattice), while F- ions in the La plane (F2,3) remained immobile. No significant anisotropy of F1 ionic diffusion within the layers and along the c-axis was found; with both being of the order of 6 x 10-14m2/s at 400K. From the nuclear magnetic resonance spectra, it was clear that F1 mobility was highly heterogeneous. The motional disorder could be described well by a broad distribution of correlation times. This had a shape which was close to a log-Gaussian function, and reflected the potential energy landscape in the superionic state. The variation in the centre position and width of the distribution, as a function of temperature, differed from Arrhenius law behavior. Therefore, ionic mobility at the microscopic scale could not be considered to be a process which was only thermally activated. The application of molecular dynamics techniques showed that the presence of vacancies could lead to marked changes in the potential energies, and supported the suggestion 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 - Condensed Matter, 1997, 9[43], 9275-87

Table 165

Diffusivity of F in Molten Li2BeF4