Selenidostannates and a Silver Selenidostannate Synthesized in Deep Eutectic Solvents: Crystal Structures and Thermochromic Study

Deep eutectic solvents (DESs) have been adopted as reaction media for solvothermal synthesis of crystal materials. In the present work, we extended the scope of DESs in chalcogenidometalate preparation by including dimethylamine, ethylamine, and trimethylamine hydrochlorides and synthesized a series of novel Sn-Se and Ag-Sn-Se compounds: i.e., [NH₂(CH₃)₂]₂Sn₃Se₇·0.5NH­(CH₃)₂ (1), [NH₄]₂Sn₄Se₉ (2), [NH₃C₂H₅]₂Sn₃Se₇ (3), and [NH₄]₃AgSn₃Se₈ (4). Compounds 1 and 3 possess honeycomb lamellar [Sn₃Se₇]_n^2n– structures featuring large hexagonal windows, while compound 2 features a rare [Sn₄Se₉]_n^2n– anionic layer consisting of tetrameric {Sn₄Se₁₀} clusters as secondary building units (SBUs). Compound 4 comprises infinite [AgSn₃Se₈]_n^3n– chains built by {Sn₃Se₈} units with Ag⁺ linkers, and it represents the first heterometallic chalcogenide synthesized in DESs. The organic ammonium cations of halide salts or in situ formed ammonium cations from the decomposition of urea act as templating agents for the formation of the inorganic frameworks. Compound 4 exhibits a marked thermochromic performance in the visible light range owing to the negative temperature dependence of its band gap (E_g = 2.305–2.119 eV in the range of 100–450 K). The gold−dark red−gold color change is highly reversible in five rounds of heating and cooling, without any phase transition of the material, shedding light on the consequent device innovations.