A high temperature superionic phase of CsSn2F5

The compound CsSn2F5 has been investigated over the temperature range from ambient to 545 K using differential scanning calorimetry, impedance spectroscopy and neutron powder diffraction methods. A first-order phase transition is observed from DSC measurements at 510(2) K, to a phase possessing a high ionic conductivity (σ∼2.5×10−2 Ω−1 cm−1 at 520 K). The crystal structure of the high temperature superionic phase (labelled α) has been determined to be tetragonal (space group I4/mmm, a=4.2606(10) Å, c=19.739(5) Å and Z=2) in which the cations form layers perpendicular to the [001] direction, with a stacking sequence CsSnSnCsSnSn… All the anions are located in two partially occupied sites in the gap between the Cs and Sn layers, whilst the space between the Sn cations is empty, due to the orientation of the lone-pair electrons associated with the Sn2+. The structure of α-CsSn2F5 is discussed in relation to two other layered F− conducting superionic phases containing Sn2+ cations, α-RbSn2F5 and α-PbSnF4 and, to facilitate this comparison, an improved structural characterisation of the former is also presented. The wider issue of the role of lone-pair cations such as Sn2+ in promoting dynamic disorder within an anion substructure is also briefly addressed.