A high temperature superionic phase of CsSn2F5

Journal article, 2010

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 (sigma similar to 2.5 x 10(-2)Omega(-1) cm(-1) at 520 K). The crystal structure of the high temperature superionic phase (labelled alpha) has been determined to be tetragonal (space group 14/mmm, a=4.2606(10)angstrom, c=19.739(5)angstrom 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 alpha-CsSn2F5 is discussed in relation to two other layered F- conducting superionic phases containing Sn cations, alpha-RbSn2F5 and alpha-PbSriF(4) 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. (C) 2009 Elsevier Inc. All rights reserved.