A 1- and 2-D 19F MAS NMR Study of Fluoride-Ion Mobility in α PbF2

One- and two-dimensional 19 F high-speed MAS NMR are used to probe motion of the fluorine sublattices in the fluoride-ion conductors α PbF 2 and potassium fluoride-doped α PbF 2 . The two crystallographic sites F(1) and F(2) are resolved in the 19 F spectrum of the pure material and are assigned on the basis of their 207 Pb− 19 F J-coupling. A resonance from fluoride-ions jumping rapidly between the two sites is also observed above 120 °C, which increases in intensity as the temperature is raised. The resonance from the mobile fluoride-ions is observed at room temperature for the α PbF 2 sample containing potassium impurities and for samples that have been intentionally doped with KF by direct reaction of KF and PbF 2 . The correlation times of the rigid and mobile fluoride-ions in these samples differ by more than 2 orders of magnitude, and 2-D magnetization exchange methods show that the exchange between these two sets of fluoride-ions is negligible. The vacancies produced by potassium doping appear to remain closely associated with the potassium defects at low temperatures, and the mobile fluoride-ions at these temperatures are assigned to fluoride-ions near the potassium defects. In contrast, in the pure, or more uniformly potassium-doped, materials, the vacancies are more uniformly distributed over the solid, resulting in spectra with a narrower range of correlation times for fluoride-ion motion. Finally, a low activation energy conduction pathway between F(1) and F(2) sites along the y -axis is proposed to rationalize the rapid F(1)↔F(2) fluoride-ion diffusion.