High Anionic Conductive Form of Pb_xSn_2-xF₄

A high anionic conductivity of ∼3.5 × 10^-3 S cm^-1 at room temperature is achieved for Pb_xSn_2-xF₄ (x = 1.21) obtained by annealing a mechanically milled PbF₂/SnF₂ mixture at 400 °C. The observed synchrotron X-ray diffraction patterns indicate formation of a new tetragonal phase at x = 1.1-1.3. The Rietveld analysis of the neutron diffraction patterns leads to a unique structure consisting of two alternating layers, namely, a double Pb layer and a triple layer, each flanked by a single Sn layer. As the Rietveld analysis does not fully converge, the authors further apply high-resolution solid-state NMR (¹⁹F, ¹¹⁹Sn, and ²⁰⁷Pb) to confirm the structure. Further, the ¹⁹F-²⁰⁷Pb cross-polarization experiment shows that most F^- ions, except for those that lie between the double Pb layers, contribute to its high ionic conductivity. The high conductivity is also attributed to structural flexibility of the triple Pb layers, indicated by temperature-dependent ²⁰⁷Pb NMR spectra.