Sn-K and Pb-L3 EXAFS, X-Ray Diffraction, and 119Sn Mössbauer Spectroscopic Studies of Ordered β-PbSnF4 and Disordered Pb1-xSnxF2 (x = 0.3, 0.4) Solid Solutions and PbSn4F10: High Performance Fluoride Ion Conductors

Fully and partly disordered superionic conductors in the PbF2/SnF2 system have been studied using X-ray powder diffraction, 119Sn Mössbauer spectroscopy, and Pb-L3 and Sn-K EXAFS (extended X-ray absorption fine structure). X-ray diffraction is seriously limited because of the large degree of disorder at the metal site. The solid solution Pb1-xSnxF2 (x = 0-0.30) is fully disordered as is the stoichiometric compound PbSn4F10 (x = 0.80). On the other hand, some degree of ordering occurs in Pb0.6Sn0.4F2 (x = 0.40) and β-PbSnF4 (x = 0.50). 119 Sn Mössbauer spectra show that the tin(II) lone pair is stereoactive in these materials. Therefore, the tin environment is highly distorted and its nonbonded electron pair is not a charge carrier in the conduction mechanism. The Sn-K and Pb-L3 EXAFS spectra indicate average first shell distances of RSn-F = 2.12(1) Å and RPb-F = 2.55(1) Å. There is a clear second shell Pb-Pb signal in the Pb-L3 EXAFS but no higher shell signal in the Sn-K EXAFS, indicating a higher degree of disorder and a lower rigidity of next-neighbor fluorine shells around Sn than around Pb. The results are discussed in relation to previous studies of ordered MSnF4 fluoride ionic conductors and to proposed mechanisms for ionic conduction in these superionic materials.