Fluoride-Ion Conductors Derived from the Fluorite Type

Abstract

Mossbauer spectroscopy has seldom been used for the study of ionic conductors. When tin(II) is incorporated in the best binary fluoride ion conductors, namely in fluorite type MF2, various new materials are formed, which have a structure related to the fluorite type, and their fluoride ion conductivity is enhanced by up to 103. Most of these new conducting materials show order/disorder phenomena. PbSnF4, which is the highest performance fluoride ion conductor, can exist in several polymorphic forms, and has a very complex system of phase transitions. SnF2, can be incorporated in BaCIF to form a wide Ba1-xSnxCl1+yF1-y solid solution, which is fully disordered in terms of metals and of excess anion of one kind relative to the other. Tin-119 Mossbauer spectroscopy can be an invaluable technique for probing the tin sites. It has made possible the interpretation of the structure of the disordered phases. In addition, the information obtained about the valence electronic structure of tin(II) makes it very easy to distinguish between: (i) a stereoactive lone pair that is located on a tin hybrid orbital, and therefore cannot be a charge carrier, and (ii) a potentially mobile non-stereoactive lone pair located on the unhybridized 5s orbital of tin.