Abstract
Two-dimensional fluoride ion conductor KSn
2F
5 undergoes a first-order phase transition to a superionic state at
T
tr=428 K associated with Δ
S
tr=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
T
tr 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.