Resolving the Different Dynamics of the Fluorine Sublattices in the Anionic Conductor BaSnF4 by Using High-Resolution MAS NMR Techniques

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Abstract
19F and 119Sn MAS NMR spectroscopy have been used to investigate the fluoride ion conductor, BaSnF4, a member of the MSnF4 family of fluorite-related anionic conductors containing double layers of Sn2+ and M2+ cations. Two fluorine sublattices were observed by 19F MAS NMR, which could be assigned to specific sites in the lattice. The first sublattice is due to fluorine atoms located in Ba2+ double layers and is rigid on the MAS NMR time scale at room temperature. The second sublattice comprises the fluoride ions between the Ba2+ and Sn2+ layers, and the few fluorine atoms that inhabit the Sn2+−Sn2+ double layers. These ions are in rapid exchange with each other, and an extremely short correlation time τC for the motion of these ions of -5 s is obtained at −100 °C. T1 measurements indicate that τC approaches 10-8 s at room temperature. 19F-to-119Sn cross-polarization (CP) experiments confirmed the assignments of the resonances, and that the fluorine atoms located next to the tin atoms are extremely mobile at room temperature (and thus do not contribute to the CP process). Two-dimensional 19F exchange experiments showed that exchange between the rigid and mobile lattice does occur, but at a much slower rate (τC ≈ 10 ms at 250 °C). Low-temperature 19F MAS and 19F-to-119Sn CP NMR spectra demonstrate that the motion of the fluoride ions has almost completely frozen out by −150 °C. The results are consistent with rapid two-dimensional (anisotropic) conductivity involving the fluoride ions between the Ba and Sn layers. Conductivity in three dimensions requires hops between the ions in the BaF2-like layers and the mobile ions. This process does occur, but with exchange rates that are at least 6−7 orders of magnitude slower.

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