Reaction between LiF and MF5 (M = Ta, Nb) in a fluorine atmosphere

LiMF6 (M = Ta, Nb) was prepared by the reaction between LiF and MF5 (M = Ta, Nb) in F2 gas. Pure LiMF6 (M = Ta, Nb) salts were obtained by using the reaction at temperatures higher than 473 K under 80 kPa (F2) for 24 h. The x values in LiMFx (M = Ta, Nb) were confirmed as 5.7–6.0 by XRD–Rietveld analysis. Results showed that LiMF6 (M = Ta, Nb) has a trigonal structure ( R 3 ¯ , Z = 3). The respective lattice parameters of LiTaF6 and LiNbF6 are a0 = 0.533 nm, c0 = 1.362 and a0 = 0.532 nm, c0 = 1.360. The equivalent conductivities of both LiMF6 (M = Ta, Nb) in propylene carbonate (PC) are equal at 15.2 Ω−1 cm2 mol−1 at 0.01 mol dm−3. The electrochemical potential window of TaF6− is 7.0 V, which is 0.4 and 0.2 V wider, respectively, than those of BF4− and PF6−.