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.