Directed Dehydration as Synthetic Tool for Generation of a New Na${}_4$SnS${}_4$ Polymorph: Crystal Structure, Na${}^{+}$ Conductivity, and Influence of Sb‐Substitution

We present the convenient synthesis and characterization of the new ternary thiostannate Na${}_4$SnS${}_4$4 (space group I4${}_1$/acd ) by directed removal of crystal water molecules from Na${}_4$SnS${}_4$⋅14 H${}_2$O. The compound represents a new kinetically stable polymorph of Na${}_4$SnS${}_4$, which is transformed into the known, thermodynamically stable form (space group P$\bar{4}$2${}_1$c) at elevated temperatures. Thermal co-decomposition of mixtures with Na${}_3$SbS${}_4$⋅9 H${}_2$O generates solid solution products Na${}_4-x$Sn${}_{1-x}$Sb${}_x$S${}_4$ (x=0.01, 0.10) isostructural to the new polymorph (x=0). Incorporation of Sb${}^{5+}$ affects the bonding and local structural situation noticeably evidenced by X-ray diffraction, ${}_{119}$Sn and ${}_{23}$Na NMR, and ${}_{119}$Sn Mössbauer spectroscopy. Electrochemical impedance spectroscopy demonstrates an enormous improvement of the ionic conductivity with increasing Sb content for the solid solution (σ${}_{25°}$C=2×10${}_{-3}$, 2×10${}_{-2}$, and 0.1 mS cm${}_{-1}$ for x=0, 0.01, and 0.10), being several orders of magnitude higher than for the known Na${}_4$SnS${}_4$ polymorph.