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.