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}$, which is transformed into the known, thermodynamically stable form (space group P$\overline{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.