Structural phase transition of the two-dimensional fluoride ion conductor KSn2F5 studied by X-ray diffraction

Two-dimensional fluoride ion conductor KSn2F5 undergoes a first-order phase transition to a superionic state at Ttr=428 K associated with ΔStr=6.3 J K−1 mol−1. The structural changes in the temperature range from 200 to 450 K, including the superionic phase transition, were investigated by means of single-crystal X-ray analysis and Rietveld analysis of the powder data. The intrinsic fluoride ion vacancies formed in the anionic layer contribute to the high ionic conductivity at Phase II (10−5 S cm−1 at 300 K). On the other hand, a two-dimensional average structure appears at Phase I due to the formation of the dynamically disordered F− sites in the anionic layer. The step-wise expansion in the ab plane at Ttr also supports this highly disordered structure. The charge density distribution derived from the MEM analysis at 450 K clearly showed a two-dimensional conduction path.