Mossbauer spectroscopy has been seldom used for the characterization of ionic conductors. However, since the introduction of divalent tin in MF2 fluorites (M = Sr, Pb and Ba) to form MSnF4, PbSn4F10 or the Pbl.,Sn,Fz solid solution, all of which have structures closely related to the fluorite type, results in an enhancement of the fluoride ion conductivity by up to three orders of magnitude, and since Sn-119 is the second best Mossbauer nuclide, it seems that the Mossbauer technique could provide useful information about how tin(IT) modifies the fluorite structure and leads to such a tremendous enhancement of the fluoride ion mobility. The MF2/SnF2 systems contain some number of materials that show order/disorder phenomena (between M and Sn, and also between different fluorine atoms) which make it difficult to understand them from diffraction data only. Mossbauer spectroscopy has been invaluable in helping understand the local structure at tin. By probing the valence electronic structure of tin, we can also make predictions on the possible long range mobility of the tin(II) non-bonded electron pair, which would make the material an electronic conductor or a mixed conductor.
