Synthesis and X-ray-graphical characteristics of the MеSn₂F₅ (Mе = Na, K, Rb, Cs) fluoride-ion conductors

Objectives. Pentafluorodistannates of alkali metals are promising materials for use as electrolytes in fluoride-ion batteries due to their electrophysical properties, such as high fluoride-ion conductivity. This work aims to synthesize crystals of alkali metals MeSn₂F₅ (Me = Na, K, Rb, Cs), carry out X-ray diffraction studies on them, and investigate the possibility of obtaining lithium fluorostannates.

Methods. Supersaturated aqueous solutions were employed to synthesize the crystals. The X-ray diffraction (XRD) analysis was carried out.

Results. Oversaturated solutions yield microcrystalline powders of sodium, potassium, rubidium, and cesium pentafluorodistannates. The presence of a single-phase was confirmed by XRD analysis of the powders corresponding to the MеSn₂F₅ (Mе = Na, K, Rb, Cs) composition. XRD data analysis and literature indicated that MеSn₂F₅ (Mе = K, Rb, Cs) have a fluorite-like structure, with the cations forming three-layer closest packing. The RbSn₂F₅ compound was discovered to be isostructural to KSn₂F₅. Based on this discovery, RbSn₂F₅ was reindexed to a hexagonal unit cell with parameters a = 7.40(3) Å, с = 10.12(6) Å (KSn₂F₅ P3, a = 7.29(3) Å, с = 9.86(2) Å). The CsSn₂F₅ compound was reindexed to a monoclinic unit cell (a = 10.03(4) Å, b = 5.92(7) Å, c = 11.96(9) Å, β = 107.4(5)°). A crystallochemical analysis of the pentafluorodistannates was carried out, and common structural motifs were discovered. The motifs are similar to lead tetrafluorostannate PbSnF₄, the best fluoride-ion conductor. The effect of the pentafluorodistannates structures on the ionic conductivity is considered. The LiF–SnF₂ system contains no compounds; the compositions were obtained by melting the original fluorides.

Conclusions. MеSn₂F₅ (Mе = Na, K, Rb, Cs) were synthesized and investigated by XRD analysis. The structural characteristics of the RbSn2F5 and CsSn₂F₅ compounds have been redefined. The crystallochemical structure is analyzed in relation to the electrophysical properties of the alkali metal pentafluorodistannates. Pentafluorodistannates MеSn₂F₅ (Mе = K, Rb, Cs) have a fluorite-like structural motif with cubic parameters а = 5.694 Å (KSn₂F₅), а = 5.846 Å (RbSn₂F₅), а = 6.100 Å (CsSn₂F₅), with the cations forming three-layer closest packing. The cationic layers alternate like Me–Sn–Sn–Me (Mе = K, Rb, Cs). For KSn2F5 and RbSn₂F₅, they are normal to the three-fold axis and normal to the four-fold axis in the case of CsSn₂F₅.

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