Ionic Conductivity of the new Fluoride-Ion Conductor Casn 2 F 6
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IONIC CONDUCTIVITY OF THE NEW FLUORIDE-ION CONDUCTOR CaSn2F6 Michael F. BELL, Georges DÉNÉS1 and Zhimeng ZHU Laboratory of Solid State Chemistry and Mössbauer spectroscopy, Laboratories for Inorganic Materials, Department of Chemistry and Biochemistry, Concordia University, Montréal, Québec, Canada ABSTRACT Metastable CaSn2F6 has been prepared for the first time and characterized. It is a well crystalline material that leaches SnF2 in water to give the microcrystalline fluorite-type Ca1-xSnxF2 solid solution. In both materials, tin(II) is covalently bonded to fluorine, and thus carries a stereoactive non-bonding electronic pair. The electrical conductivity of CaSn2F6 was measured by the complex impedance method. The CaSn2F6 material was found to be a mixed conductor (τi = 0.50), with a F- conductivity a little below that of α-SnF2. On heating to 250oC, it decomposes irreversibly to give SnF2 and probably amorphous CaF2 (undetected). INTRODUCTION The most efficient fluoride-ion conductors contain divalent tin and/or have a structure derived from that of the CaF2 fluorite-type structure. The conductivity of tetrameric α-SnF2 is higher than that of β-PbF2, the best fluoride-ion conductor among the fluorite type, however, at γ transition, the conductivity of α-SnF2 decreases below that of β-PbF2 [1]. The MSnF4 the α materials (M = Ba and Pb) have a conductivity three orders of magnitude higher than that of the corresponding MF2, with PbSnF4, being the very best [2 & 3]. The first preparation of MF2/SnF2 materials was carried out by Donaldson and Senior, who prepared PbSnF4, MSn2F6 and MSn4F10 (M = Sr, Ba and Pb) [4]. However, these authors were unable to obtain well defined materials in the CaF2/SnF2 system, and precipitation reactions between calcium nitrate and tin(II) fluoride in aqueous solution were reported to be not reproducible and give semi-amorphous materials with a variable stoichiometry. No reaction was obtained by one of us (GD) when CaF2 and SnF2 were heated together [5]. The preparation, structures and phase transitions of new materials in the MF2/SnF2 systems (M = Sr, Ba and Pb) was later studied in details by Dénès et al [5]. It was found that many of the phases obtained have a structure related to the fluorite-type, some with M/Sn order, while others are partially or fully disordered [6 & 7]. It was also found that PbSnF4 undergoes many phase transitions , making it one of the most complex material known [8]. In the present work, we report the first preparation of crystalline CaSn2F6, its characterization, and the study of its electrical properties. It was found to have a conductivity slightly lower than that of α-SnF2, and higher than that of γ-SnF2, with a transport number for ions, τi = 0.50, and to decompose irreversibly when heated up to 270oC.
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To whom all correspondence should be addressed: [email protected]
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EXPERIMENTAL PROCEDURES CaSn2F6 was obtained by precipitation when a 1.5M solution of Ca(NO3)2.2H2O (Fisher, 99%) was added on stirring to a 1.5M solution of
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