$^\mathsf{207}$Pb MAS NMR and conductivity identified anomalous phase transition in nanostructured PbF$_\mathsf{2}$

Abstract.

Lead fluoride, a superionic conductor was prepared in its nanostructured form by Inert Gas Condensation Technique (IGCT) using an Ultra High Vacuum (UHV) chamber. The average grain size was found to be in the range 9 to 43 nm. The existence of mixed phases ($\alpha$ and $\beta$-PbF₂) was identified using XRD. Solid state ²⁰⁷Pb MAS NMR was carried to average out the dipolar interaction and the resultant isotropic peaks were assigned to the corresponding phases. At high spinning frequencies, one isotropic peak emerged from the contribution of the grain boundary region. The relative intensity of this peak is reduced as the grain size is increased, independent of the concentration of the phases. This is related to the fact that the volume fraction of grain boundary atoms in nanostructured materials increases with the reduction of grain size. The width of the NMR resonance peak is found to be reduced as the grain size goes down. The structural phase transformations were identified at two different temperature regions. The first phase transformation from $\beta$ to $\alpha$ phase in the annealing temperature range 573 K to 623 K is attributed to some anomalies related to the material microstructure and this has not been reported in earlier literatures. The second phase transformation from $\alpha$ to $\beta$ in the temperature range 623 K to 673 K is similar to the already reported transformation. Electrical conductivity $\sigma$, of the samples was obtained from the complex impedance spectroscopy studies. Conduction species was identified as F^- ion through anion vacancies. The magnitude of the conductivity varied according to the dominant phase available when the grain size is higher. But at lower grain sizes below 20 nm, it shows enhanced conductivity that is attributed to the grain size effect. The NMR and conductivity data have jointly supported the anomalous phase transition at the annealing temperature of 623 K.