THE DISTRIBUTION OF MOTIONAL CORRELATION TIMES IN SUPERIONIC CONDUCTORS - F-19 NUCLEAR-MAGNETIC-RESONANCE OF TYSONITE-LIKE LAF3

Fluorine mobility at different structural positions in monocrystalline LaF3 with the tysonite structure is analysed using F-19 NMR line-shap e analysis. The method is sensitive to ionic exchange with correlation times in the range 10(-6)-10(-3) s. For the temperature range between 240 K and 400 K the motion is restricted mainly to the F- ions in the fluorine layers perpendicular to the main symmetry axis (the F-1 subl attice), while F- ions in the La plane (F-2,F-3) remain immobile. No s ignificant anisotropy of the F-1-ionic diffusion within the layers and along the c-axis is found (D-parallel to c approximate to D-perpendic ular to c approximate to 6 x 10(-14) m(2) s(-1) at 400 K). From NMR sp ectra it is clear that Fl mobility is strongly heterogeneous. The moti onal disorder can be described well by a broad distribution of correla tion times, G(tau), which has a shape close to a log-Gaussian function and reflects the potential energy landscape in the superionic state. The variation Of the centre position and width of G(tau) with temperat ure differs from an Arrhenius law behaviour. Ionic mobility on the mic roscopical scale, therefore, cannot be considered a process which is a ctivated only thermally. Applying MD techniques shows that the presenc e of vacancies may lead to pronounced changes of the potential energie s, and supports the idea that there is a distribution of activation en ergies.