Fluoride Ion Diffusion in MSn2F5 (M+ NH4+, Rb+, Cs+) Studied by 1H and 19F Nuclear Magnetic Relaxation and Electrical Conductivity

Fluoride ion diffusion in NH4Sn2F5, RbSn2F5, and CsSn2F5 was investigated by measuring the electrical conductivity (σ) and nuclear magnetic relaxation times (T1, T1ϱ, and T2) of 1H and 19F nuclei. The frequency dependence of T1 (1H) shows a logarithmic divergence in the high-temperature phase of NH4Sn2F5, indicating that the fluoride ion diffusion in this phase is two-dimensional. In the room-temperature phases of NH4Sn2F5 and RbSn2F5, this two-dimensionality of the diffusion is weakened. An asymmetric T1ϱ minimum observed for NH4Sn2F5 and RbSn2F5 was analyzed using an analytical expression, τ ln {1 + (ωτ)−1-β}, for the spectral density of the 2D fluoride ion diffusion, and the β parameter introduced explain the asymmetry of the T1ϱ minimum and a large difference between apparent activation energies deduced from T1 and T2 data. The activation energies determined from the relaxation times in the high-temperature limit agree well with those from the σ measurements.