Room-Temperature Solid-State Transformation of Na4SnS4•14H2O into Na4Sn2S6 • 5H2O: An Unusual Epitaxial Reaction Including Bond Formation, Mass Transport, and Ionic Conductivity

A highly unusual solid-state epitaxy-induced phase transformation of Na4SnS4 center dot 14H(2)O(I) into Na4Sn2S6 center dot 5H(2)O (II) occurs at room temperature. Ab initio molecular dynamics (AIMD) simulations indicate an internal acid-base reaction to form [SnS3SH](3-) which condensates to [Sn2S6](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 [Sn2S6](4-). DFT calculations allowed assignment of all bands appearing during the transformation. X-ray diffraction and in situ H-1 NMR demonstrate a transformation within several days and yield a reaction turnover of approximate to 0.38%/h. AIMD and experimental ionic conductivity data closely follow a Vogel-Fulcher-Tammann type T dependence with D(Na)=6x 10(-14) m(2) s(-1) at T=300 K with values increasing by three orders of magnitude from -20 to +25 degrees C.