A 1- and 2-D 19F MAS NMR study of fluoride-ion mobility in a PbF2

One- and two-dimensional F-19 high-speed MAS NMR are used to probe motion of the fluorine sublattices in the fluoride-ion conductors alpha PbF2 and potassium fluoride-doped alpha PbF2. The two crystallographic sites F(1) and F(2) are resolved in the F-19 spectrum of the pure material and an assigned on the basis of their Pb-207-F-19 J-coupling. A resonance from fluoride-ions jumping rapidly between the two sites is also observed above 120 degrees C, which increases in intensity as the temperature is raised. The resonance from the mobile fluoride-ions is observed at room temperature for the alpha PbF2 sample containing potassium impurities and for samples that have been intentionally doped with KF by direct reaction of KF and PbF2. 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.