Unusual Atmospheric Water Trapping and Water Induced Reversible Restacking of 2D Gallium Sulfide Layers in NaGaS₂ Formed by Supertetrahedral Building Unit

Two new ternary materials NaGaS2 (1) and the Fe-doped phase of NaGaS2, NaFe0.135Ga0.865S2 (2), have been synthesized by employing polysulfide flux. Single crystal XRD analyses of 1 and 2 show that the structure is built up of adamantane-like Ga4S10 super tetrahedral fundamental building units. These admantane-like units are connected through their corners to form [GaS2]∞– layers that are stacked one over the other with Na ions residing in between the layers to balance the charge. Both the materials have the remarkable ability to absorb atmospheric water molecules and moisture from undried solvents as verified by TG analysis and FT-IR and XPS studies. The process of water absorption leads to stable distinct material NaGaS2·∼H2O (1·H2O) and NaFe0.135Ga0.865S2·∼H2O (2·H2O) with restacked layers different from original crystal structure. This structural transformation is reversible as the transformed structures 1·H2O and 2·H2O can be returned to their original structures 1 and 2, respectively, upon heating. DFT calculation study reveals that a spontaneous exergonic hydration reaction takes place as outlined in NaGaS2 + H2O → NaGaS2·H2O with the energy release, ΔE of −73.9 kJ mol–1. DFT calculation predicts an increase in the unit cell parameters of b and c directions and shrinkage along the a direction of hydrated phase 1·H2O with an overall volume increase of 36.6%. Structural transformation affects their physical properties as the pristine compound 1 possess Na+ ion conductivity of 2.88 × 10–7 S cm–1 at 22 °C, whereas the hydrated compound 1·H2O displays ∼40 times increased ion conductivity of 1.25 × 10–5 S cm–1 at the same temperature. DRS studies show very similar optical band gaps of ∼4 eV for compounds 1 and 1·H2O, respectively, in reasonable agreement with the DFT(HSE) band gap estimation but more than 1 eV above the DFT(PBE)-predicted band gaps of ∼2.4 eV. A sorption study indicates selective adsorption of water over MeOH, EtOH, and CH3CN with a maximum water uptake of 2.6 H2O per formula unit at P/P0 = 0.9. A Karl Fischer titration study shows that NaGaS2 (1) is certainly capable of adsorbing water from wet methanol and can be useful as a fast desiccating agent.