Directed Dehydration of Na$_4$Sn$_2$S$_6$ ⋅ 5H$_2$O Generates the New Compound Na$_4$Sn$_2$S$_6$: Crystal Structure and Selected Properties

The new thiostannate Na$_4$Sn$_2$S$_6$ was prepared by directed crystal water removal from the hydrate Na$_4$Sn$_2$S$_6$ ⋅ 5H$_2$O at moderate temperatures. While the structure of the hydrate comprises isolated [Sn$_2$S$_6$]$^{4−}$ anions, that of the anhydrate contains linear chains composed of corner-sharing SnS4 tetrahedra, a structural motif not known in thiostannate chemistry. This structural rearrangement requires bond-breakage in the [Sn2S6]4− anion, movements of the fragments of the opened [Sn2S6]4− anion and Sn−S−Sn bond formation. Simultaneously, the coordination environment of the Na+ cations is significantly altered and the in situ formed NaS$_5$ polyhedra are joined by corner- and edge-sharing to form a six-membered ring. Time-dependent in situ X-ray powder diffraction evidences very fast rehydration into Na$_4$Sn$_2$S$_6$ ⋅ 5H$_2$O during storage in air atmosphere, but recovery of the initial crystallinity requires several days. Impedance spectroscopy demonstrates a mediocre room-temperature Na$^+$ ion conductivity of 0.31 μS cm$^{−1}$ and an activation energy for ionic transport of E$_a$=0.75 eV.