Variations of BaSnF₄ fast ion conductor with the method of preparation and temperature

Abstract

Ionic conductors are solids that have a large number of defects and easy pathways that make it possible for ions to move over long distances in an electric field. In order to be mobile an ion must be small and have a low charge. The fluoride ion is the most mobile anion. The highest performance fluoride ion conductors contain divalent tin, and have a highly layered crystal structure related to the CaF₂ fluorite type. BaSnF₄ has the α-PbSnF₄ structure, which is a √2/2 × √2/2 × 2 superstructure of the fluorite type, where the tetragonal unit-cell and the value of the a and b parameters being equal to half the diagonals of the (a,b) face of fluorite are due to the loss of the F Bravais lattice, and the Sn Sn Ba Ba order along the c parameter is at the origin of the doubling of the c parameter. The BaSnF₄ material was prepared first by Dénès et al. (C. R. Acad. Paris C, 280: 831, 1975), and its superionic properties were characterized by Dénès et al. (Solid State Ion., 13: 213, 1984). It was found to have a conductivity three orders of magnitude higher than that of BaF₂, with an ionic conduction rate τ _i > 0.99. No BaSnF₄ was obtained by the aqueous medium, when aqueous solutions of SnF₂ and Ba(NO₃)₂ are mixed together; BaSn₂F₆ was obtained instead. In a new development of this work, BaSnF₄ has been obtained by the wet method for the first time. X-ray powder diffraction showed that the BaSnF₄ phase obtained by the wet method varies substantially from one sample to another: (a) signification variations of the c parameter of the tetragonal unit-cell reveals that the interlayer distance is sensitive to the leaching conditions, possibly because some of the leached ions remain in the interlayer spacing; (b) large variations of the crystallite dimensions and, as a result of the two-dimensionality of the structure, a strong crystallite dimension anisotropy are observed, with d∥ < d⊥, where d∥ and d⊥ are the crystallite dimensions parallel to the four-fold main axis, and perpendicular to it, respectively, showing that the layers are very thin and the interlayer interactions are very weak. Variable temperature Mössbauer spectroscopy showed an unusual large variation of the quadrupole splitting with temperature. A tentative explanation based on unusually large bond angles has been proposed.