Room-Temperature Solid-State Transformation of Na4 SnS4 ⋅ 14H2 O into Na4 Sn2 S6 ⋅ 5H2 O: An Unusual Epitaxial Reaction Including Bond Formation, Mass Transport, and Ionic Conductivity

A highly unusual solid-state epitaxy-induced phase transformation of Na₄ SnS₄  ⋅ 14H₂ O (I) into Na₄ Sn₂ S₆  ⋅ 5H₂ O (II) occurs at room temperature. Ab initio molecular dynamics (AIMD) simulations indicate an internal acid-base reaction to form [SnS₃ SH]^3- which condensates to [Sn₂ S₆ ]^4- . The reaction involves a complex sequence of O-H bond cleavage, S^2- protonation, Sn-S bond formation and diffusion of various species while preserving the crystal morphology. In situ Raman and IR spectroscopy evidence the formation of [Sn₂ S₆ ]^4- . DFT calculations allowed assignment of all bands appearing during the transformation. X-ray diffraction and in situ ¹ H NMR demonstrate a transformation within several days and yield a reaction turnover of ≈0.38 %/h. AIMD and experimental ionic conductivity data closely follow a Vogel-Fulcher-Tammann type T dependence with D(Na)=6×10^-14  m²  s^-1 at T=300 K with values increasing by three orders of magnitude from -20 to +25 °C.

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 Na₄SnS₄ (space group I41/acd ) by directed removal of crystal water molecules from Na₄SnS₄⋅14 H₂O. The compound represents a new kinetically stable polymorph of Na₄SnS₄, which is transformed into the known, thermodynamically stable form (space group P4‾21c ) at elevated temperatures. Thermal co‐decomposition of mixtures with Na₃SbS₄⋅9 H₂O generates solid solution products Na_4−xSn_1−xSb_xS₄ (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, ¹¹⁹Sn and ²³Na NMR, and ¹¹⁹Sn 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 Na₄SnS₄ polymorph.

Synthesis and Characterization of a Rare Transition-Metal Oxothiostannate and Investigation of Its Photocatalytic Properties

The new transition-metal oxothiostannate [Ni(cyclen)(H₂O)₂]₄[Sn₁₀S₂₀O₄]·∼13H₂O (1) was prepared under hydrothermal conditions using Na₄SnS₄·14H₂O as the precursor in the presence of [Ni(cyclen)(H₂O)₂](ClO₄)₂·H₂O. Compound 1 comprises the [Sn₁₀S₂₀O₄]^8- anion constructed by the T3-type supertetrahedron [Sn₁₀S₂₀] and the [Sn₁₀O₄] anti-T2 cluster. Channels host the H₂O molecules, and the sample can be reversibly dehydrated and rehydrated without significantly affecting the crystallinity of the material. ¹¹⁹Sn NMR spectroscopy of an aqueous solution of Na₄SnS₄·14H₂O evidences that between 25 and 120 °C only [SnS₄]^4- and [Sn₂S₆]^4- anions are present. In further experiments, hints were found that the formation of tin oxosulfide ions depends on the Ni²⁺-centered complexes. Compound 1 exhibits promising photocatalytic properties for the visible-light-driven hydrogen evolution reaction, with 18.7 mmol·g^-1 H₂ being evolved after 3 h.