Synthesis, structure, physicochemical characterization and electronic structure of thio-lithium super ionic conductors, Li4GeS4 and Li4SnS4

Abstract Li 4 SnS 4 and Li 4 GeS 4 were synthesized by single-step, high-temperature, solid-state methods. The room temperature (296 K) structures of both compounds were determined from single crystal X-ray diffraction, and were found to be isostructural. Both room temperature structures exhibit significant variations from a 100 K structure of Li 4 SnS 4 published recently by Kaib et al. (Chem. Mater. 24 (2012) 2211–2219). Differential thermal analysis (DTA) shows that in the absence of air, both Li 4 SnS 4 and Li 4 GeS 4 are thermally stable at least up to their melting points of 858 °C (Li 4 GeS 4 ) and 958 °C (Li 4 SnS 4 ). The band gaps for Li 4 GeS 4 and Li 4 SnS 4 are 4.13 eV and 3.54 eV respectively, measured using optical diffuse reflectance spectroscopy in the UV/Vis/NIR regions. Electronic band structure calculations using density functional theory as implemented in CASTEP predict an indirect band gap for Li 4 GeS 4 and a direct band gap for Li 4 SnS 4 . The (Sn/Ge)–S bonds were determined to have significant covalent bonding character, while the Li–S bonds are primarily ionic in nature.