High Anionic Conductive Form of PbxSn2-xF4

Abstract

A high anionic conductivity of ∼3.5 × 10–3 S cm–1 at room temperature is achieved for PbxSn2–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.