Room‐Temperature Solid‐State Transformation of Na(4)SnS(4) ⋅ 14H(2)O into Na(4)Sn(2)S(6) ⋅ 5H(2)O: An Unusual Epitaxial Reaction Including Bond Formation, Mass Transport, and Ionic Conductivity

A highly unusual solid‐state epitaxy‐induced phase transformation of Na(4)SnS(4) ⋅ 14H(2)O (I) into Na(4)Sn(2)S(6) ⋅ 5H(2)O (II) occurs at room temperature. Ab initio molecular dynamics (AIMD) simulations indicate an internal acid‐base reaction to form [SnS(3)SH](3−) which condensates to [Sn(2)S(6)](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 [Sn(2)S(6)](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) m(2) s(−1) at T=300 K with values increasing by three orders of magnitude from −20 to +25 °C.