The corresponding Rietveld refinement shows that the synthesized t-BaSnF4 has partial mixed occupancy of Ba and Sn at the Sn1 site, which can further be different for various grains according to Ba1±xSn1∓xF4 (from the ideal structure of t-BaSnF4 . By taking such a distribution of t-BaSnF4 phases with different Ba/Sn occupancies at Ba1 and Sn1 sites into account, a good fit can be realized (detailed fitting model is given in Figure 2). It is observed that repeated processes of milling and annealing can improve the homogeneity of Ba and Sn incorporation (see Figure 3). However, full homogeneity in the distribution of Sn and Ba cannot be achieved by the suggested synthesis process, even after three repetitions; likewise, extending the annealing time or increasing the annealing temperature cannot relieve the asymmetry of reflections. Nevertheless, the functionality of the electrolyte phase is not influenced, and the conductivity is sufficiently high for both room temperature and elevated temperature operation of FIBs. The XRD pattern of the as-prepared BiF3 cathode composite containing t-BaSnF4 and carbon nanofibers is shown in Figure . The as-received BiF3 powder consists of two modifications, orthorhombic (space group Pnma, ~85 wt%) and cubic (space group Fm-3m, ~15 wt%), see 1. The lattice parameter a in cubic BiF3 in pristine powder is refined to be 5.8501(3) Å, in good agreement with the previously reported value (5.865±0.06 Å at room temperature ), and the corresponding reflections show a broad characteristics. Considering the rather large error of literature value we assumed that the cubic modification in the asreceived BiF3 powder may not have the ideal composition of BiF3. We hypothesized a compositional fluctuation due to a combination of vacancies on fluorine sites and oxygen incorporation. This is consistent with previous reports on Bismuth oxidefluoride denoted as BiF3-2xOx (0.41 ≤ x ≤ 0.52), which also crystallizes in the cubic space group Fm-3m. For x = 0.51 , the lattice parameter a is ~5.85 Å and it increases with increasing fluorine content.