Fluoride ion dynamics in nanocrystalline α-PbF₂: On the tremendous impact of structural disorder on F⁻ anion hopping in poor ion conductors

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.