Directed Dehydration as Synthetic Tool for Generation of a New Na₄SnS₄ Polymorph: Crystal Structure, Na⁺ Conductivity, and Influence of Sb‐Substitution

Abstract

We present the convenient synthesis and characterization of the new ternary thiostannate Na₄ SnS₄ (space group <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>I</mml:mi> <mml:msub><mml:mn>4</mml:mn> <mml:mn>1</mml:mn></mml:msub> <mml:mo>/</mml:mo> <mml:mi>a</mml:mi> <mml:mi>c</mml:mi> <mml:mi>d</mml:mi></mml:mrow> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>P</mml:mi> <mml:mover><mml:mn>4</mml:mn> <mml:mo>‾</mml:mo></mml:mover> <mml:msub><mml:mn>2</mml:mn> <mml:mn>1</mml:mn></mml:msub> <mml:mi>c</mml:mi></mml:mrow> </mml:math> ) 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.