Conductivity and Fluoride Ion Dynamics in α-PbSnF 4 ; 19 F Field-Cycling NMR and Diffraction Studies

Fast-field cycling 19 F NMR relaxometry has been applied to investigate fluoride ion dynamics in the layered anionic conductor PbSnF 4 . Two dynamic processes, on different timescales, were shown to drive the 19 F relaxation. By considering the temperature dependencies of the NMR, conductivity, and diffraction data, a complete mechanism for fluoride transport can for the first time be proposed. The slower process is due to anion exchange between equivalent sites (F(2)) in the conducting fluoride plane, which lie between Sn and Pb layers. This is a diffusive process related directly to the mechanism of electrical conduction. The activation barriers for this motion agree closely with those determined from the temperature dependence of the DC conductivity. The faster process is due to non-diffusive exchange between the occupied sites (F(2)) and nominally vacant sites (F(1)), which lie between Sn planes. While this process does not directly limit conductivity, it generates vacancies on the F(2) sites and partial occupancy of the F(1) sites. Above 340 K, the fast process shows an increase in activation energy, as increased occupancy of F(1) sites necessitates the formation of a Frenkel defect on a significant proportion of the F(1) sites prior to an F(2)−F(1) jump. In this temperature range, the slow process shows a decrease in the activation energy, also observed in the conductivity data, due to increased numbers of vacancies in the F(2) sites which provide additional diffusive pathways through the conducting fluoride plane. The results demonstrate that α-PbSnF 4 is essentially a two-dimensional (anisotropic) conductor, in which nondiffusive fluoride exchange into sites normal to the conducting plane provides a high population of vacancies within the conducting plane, resulting in unusually high conductivity.