Solid-State 19F MAS NMR Investigation of Fluoride Ion Mobility in Lead Fluorides: Correlation with Anionic Conductivity
Résumé
A study of the fluoride ion mobility in Pb5Ga3F19 and β-Pb2ZnF6, combining data from high-resolution magic-angle-spinning variable-temperature one-dimensional solid-state 19F NMR, two-dimensional exchange NMR experiments, temperature-controlled X-ray diffraction and complex impedance spectroscopy measurements, is presented. In Pb5Ga3F19, a mutual chemical exchange occurs, above 30 °C, between all the fluorine sites within the isolated Ga1F63− octahedra. At higher temperature, a chemical exchange between these ions and some of the free fluoride ions is observed. At least up to 186 °C, fluoride sites belonging to the Ga2F63− octahedra chains are not involved in the dynamic process, which results in the absence of anionic conductivity. In β-Pb2ZnF6, the fluoride anionic mobility depends on the sample. For T > 70 °C, a tridimensional diffusive process takes place in this Aurivillius-type phase due to chemical exchange between the overall fluoride sites, which is confirmed by the occurrence of macroscopic fluoride ionic conductivity.