The tetragonal ordered form of BaSnF₄ is of particular interest, as its ionic conductivity is high enough to enable its uses as an electrolyte in all-solid-state fluoride-ion batteries. Despite several studies related to its synthesis, structure, and fluoride-ion diffusion mechanism, reported routes often yield impurities as well as unexplained variation in the unit-cell c-axis length. Here, we report on the single-phase synthesis of t-BaSnF₄ via spark plasma sintering, a method that could be used to prepare bulk-type all-solid-state inorganic batteries in one step. By optimizing different parameters (temperature, setup features, etc.), we reached a high ionic conductivity of 5 × 10^-3 S·cm^-1 at 30 °C. In addition, we show that two main factors affect the ionic conductivity. First, on a microstructural scale, the preferential growth of crystallites along the c-axis results in a decrease of the ionic conductivity of resulting powders because of the two-dimensional (2D) fluoride-ion diffusion in this material. Second, on the atomic scale, the increase of the unit-cell c-axis length is concomitant with a decrease of the ionic conductivity. A combined neutron diffraction and ¹⁹F solid-state magic angle spinning (MAS) NMR study reveals that the observed increase of the unit-cell c-axis length is due to the partial occupancy of octahedral interstitial sites. NMR allows us to identify these interstitial sites (the F4 site) with distinct isotropic chemical shift values. Furthermore, variable-temperature ¹⁹F solid-state MAS NMR reveals that these F4-ions do not exchange with fluoride-ions (F1 and F3) that are responsible for the transport properties. Hence, the occupancy of these interstitial sites tends to lower the 2D fluoride-ion conductivity, and the unit-cell c-axis length can be used as a guideline to ensure the preparation of highly conductive samples provided that the microstructure is controlled. Overall, this study provides a novel route to prepare pure t-BaSnF₄ while establishing a better understanding of the factors affecting its transport properties.

Because of its sensitivity to the atomic scale environment, solid-state NMR offers new perspectives in terms of structural characterization, especially when applied jointly with first-principles calculations. Particularly, challenging is the study of actinide-based materials because of the electronic complexity of the actinide cations and to the hazards due to their radioactivity. Consequently, very few studies have been published in this subfield. In the present paper, we report a joint experimental-theoretical analysis of thorium tetrafluoride, ThF₄, containing a closed-shell actinide (5f⁰) cation. Its crystalline structure has been revisited in the present work using powder neutron diffraction experiments. The ¹⁹F NMR parameters of the seven F crystallographic sites have been modeled using an empirical superposition model, periodic first-principles calculations, and a cluster-based all-electron approach. On the basis of the atomic position optimized structure, a complete and unambiguous assignment of the ¹⁹F NMR resonances to the F sites has been obtained.