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 (space group I 4 1 / a c d ) 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 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.