F. Wang et Gp. Grey, A 1-D AND 2-D F-19 MAS NMR-STUDY OF FLUORIDE-ION MOBILITY IN A PBF2, Journal of the American Chemical Society, 120(5), 1998, pp. 970-980
One- and two-dimensional F-19 high-speed MAS NMR are used to probe mot
ion of the fluorine sublattices in the fluoride-ion conductors alpha P
bF2 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 mate
rial and an assigned on the basis of their Pb-207-F-19 J-coupling. A r
esonance from fluoride-ions jumping rapidly between the two sites is a
lso 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 pota
ssium impurities and for samples that have been intentionally doped wi
th 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, Th
e vacancies produced by potassium doping appear to remain closely asso
ciated 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 po
tassium-doped, materials, the vacancies are more uniformly distributed
over the solid, resulting in spectra with a narrower range of correla
tion times for fluoride-ion motion. Finally, a low activation energy c
onduction pathway between F(1) and F(2) sites along the y-axis is prop
osed to rationalize the rapid F(1)<->F(2) fluoride-ion diffusion.