Directed Dehydration as Synthetic Tool for Generation of a New Na4 SnS4 Polymorph : Crystal Structure, Na+ Conductivity, and Influence of Sb-Substitution

We present the convenient synthesis and characterization of the new ternary thiostannate Na₄ SnS₄ (space group I41/acd ) by directed removal of crystal water molecules from Na₄ SnS₄ ⋅14 H₂ O. The compound represents a new kinetically stable polymorph of Na₄ SnS₄ , which is transformed into the known, thermodynamically stable form (space group P4‾21c ) at elevated temperatures. Thermal co-decomposition of mixtures with Na₃ SbS₄ ⋅9 H₂ O generates solid solution products Na_4-x Sn_1-x Sb_x S₄ (x=0.01, 0.10) isostructural to the new polymorph (x=0). Incorporation of Sb⁵⁺ 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 Na₄ SnS₄ polymorph.