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

Abstract

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] 3À which condensates to [Sn 2 S 6 ] 4À . The reaction involves a complex sequence of OÀ H bond cleavage, S 2À protonation, SnÀ 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 ] 4À . 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 À 14 m 2 s À 1 at T = 300 K with values increasing by three orders of magnitude from À 20 to + 25 °C.

Directed Dehydration of Na4Sn2S6 ⋅ 5H2O Generates the New Compound Na4Sn2S6: Crystal Structure and Selected Properties

“…The structure of the pristine compound I (space group: P4 1 2 1 2) was previously described elsewhere with a = b = 8.45088(3) and c = 23.32912(12) Å, V = 1666.102 Å 3 and contains isolated edge-sharing double-tetrahedra [Sn 2 S 6 ] 4À (Figure S6). [50] The mechanism of the structural transformation of these double-tetrahedra into corner-sharing SnS 4 tetrahedra observed in the chains of II is not clear. But notably, I and II apparently exhibit a topotaxial relationship (cf.…”

“…The [Sn 2 S 6 ] 4À anion has the D 2h symmetry and a vibrational analysis yields Equation (1). [50] G…”

“…The results demonstrate that the crystal water molecules of I can be reversibly removed with good crystallinity after incorporation of H 2 O in the dehydrated material. Since the space group ( P 4 1 2 1 2) [50] of the pristine hydrate is enantiomorphic, the chirality of the rehydrated product could differ from the initial one, which needs to be addressed in comprehensive studies in future research. Nonetheless, small fractions of an unknown side phase are detected after storage for 163 h, which are not observed after air exposure of II for 68 h (see Figure S4).…”

“…The [Sn 2 S 6 ] 4− anion has the D 2h symmetry and a vibrational analysis yields Equation [50] $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {\Gamma}{_{{\rm vib}}}{^{{\rm D2h}}}=4\ {\rm A}{_{{\rm g}}}({\rm Ra})+2\ {\rm B}{_{{\rm 1g}}}({\rm Ra})+2\ {\rm B}{_{{\rm 2g}}}({\rm Ra})+\hfill\cr 1\ {\rm B}{_{{\rm 3g}}}({\rm Ra})+1\ {\rm A}{_{{\rm u}}}({\rm inactive})+3\ {\rm B}{_{{\rm 1u}}}({\rm IR})+\hfill\cr 2\ {\rm B}{_{{\rm 2u}}}({\rm IR})+3\ {\rm B}{_{{\rm 3u}}}({\rm IR})\hfill\cr}}$$$ …”

“…If the symmetry is reduced to C i due to the interactions with the Na + cations vibrations according to Equation (2) can be expected [50] $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {\Gamma}{_{{\rm vib}}}{^{{\rm Ci}}}=9\ {\rm A}{_{{\rm g}}}({\rm Ra})+9\ {\rm A}{_{{\rm u}}}({\rm IR})\hfill\cr}}$$$ …”