Structure and ionic transport of PbSnF4 superionic conductor

Although PbSnF4, the highest performance fluoride ion conductor, was first prepared nearly 30 years ago, structural information about it have been scarce and somewhat inadequate. We have prepared samples of tetragonal PbSnF4 and orthorhombic PbSnF4 under well defined conditions. Their characterization and the study of their transport properties are presented and interpreted in the light of the structural information available and the known phase transitions. The samples were characterized by X-ray diffraction, differential scanning calorimetry, differential thermal analysis, infrared spectroscopy and impedance methods. The transport properties of tetragonal and orthorhombic PbSnF4 are very similar. The conductivity-temperature curves upon heating from 25-200 degrees C are characterized by the gradual slope change from a region of high activation energy (similar to 55 and similar to 35 kcal/mol respectively) to that of a lower activation energy (similar to 8 kcal/mol) at around 80 degrees C. Upon cooling, the electrical conductivity is observed to be linearly dependent on temperature with an activation energy of 30 kcal/mol. There is no symmetry change at the temperature where the conductivity curves show a gradual slope change.