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.