Understanding the phase transitions and texture in superionic PbSnF4: The key to reproducible properties

Abstract

The recent use of high-performance fluoride ion conductor PbSnF4 for the fabrication of a solid state room temperature amperometric oxygen sensor implies that the materials used for its fabrication is well defined and undergoes no transformation over the lifetime of the sensor in the conditions of use. There are many ways to prepare PbSnF4, and subtle differences can lead to different PbSnF4 phases. Furthermore, moderate applications of heat and mechanical energy (ball milling), or minor changes in the composition of the reaction medium, can result in phase transitions, including order/disorder phenomena. In addition, it was also found that several phases, that appear stable, are in fact metastable and undergo transformations over prolonged periods of time. Furthermore, most phases of PbSnF4 show a considerable amount of preferred orientation due to the layered structure, causing highly anisotropic properties of polycrystalline samples. We have discovered methods for eliminating the preferential orientation of the crystallites and for enhancing it, close to the situation of a single crystal, in one direction. The adequate choice of the method of preparation, proper control of the preferred orientation, and knowledge of the phase transitions, should make possible the production of a material with stable and reliable properties.