Abstract
PbF2 has already been the subject of one of Michael Faraday's experiments that played the crucial part in the discovery of fast F− anion transport in solids. In contrast to the cubic form of PbF2, ion dynamics in orthorhombic modification, that is, α-PbF2, is rather poor at ambient conditions. Its complex crystal structure hosts two magnetically inequivalent F sites (F1 and F2), which makes it, however, a rather interesting model substance to study fundamental aspects of ion transport. Here, to contribute to a general understanding of ion dynamics in compounds crystallizing with PbCl2 (cotunnite) structure, we studied the influence of structural disorder on ion dynamics in nominally pure α-PbF2. We expect that this approach will help us to understand also the hopping mechanisms in ordered PbF2. Broadband conductivity spectroscopy and electric modulus spectroscopy showed that disorder causes the F− anion conductivity of α-PbF2 to increase by a factor of 100. In excellent agreement with this result, atomic-scale 19F high-resolution NMR spectroscopy reveals a clear increase in F1-F2 exchange shedding, at the same time, light on a controversial discussion how to interpret the 19F MAS NMR spectra of α-PbF2 in detail. In addition, a quantitative analysis of long-range ion transport is provided by broadband conductivity measurements. We show that F− anion dynamics in nanocrystalline PbF2 has to be described by an activation energy of 0.48 eV, which is significantly lower than that found for the mechanically untreated sample (0.6 eV at T > 300 K). This change in ion dynamics is reflected also by time-domain 19F NMR spectroscopy pointing to local activation barriers with values ranging from 0.17 eV to 0.31 eV in nanocrystalline α-PbF2. NMR line shape measurements indicate that the F1 site, in particular, plays an important role in overall anion exchange in the nanostructured form.
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