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

[Display omitted] •The title compounds were prepared by a high-temperature, direct-combination route.•The room temperature structure of Li4SnS4 is reported in space group Pnma.•The Li(2)S4 tetrahedra invert upon going from the RT to low temperature structure.•Electronic structure calculations indicate the Li–S bonding to be primarily ionic.•Li4GeS4 and Li4SnS4 have shown to be stable up to ∼850 and 950°C respectively. Li4SnS4 and Li4GeS4 were synthesized by single-step, high-temperature, solid-state methods. The room temperature (296K) 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 100K structure of Li4SnS4 published recently by Kaib et al. (Chem. Mater. 24 (2012) 2211–2219). Differential thermal analysis (DTA) shows that in the absence of air, both Li4SnS4 and Li4GeS4 are thermally stable at least up to their melting points of 858°C (Li4GeS4) and 958°C (Li4SnS4). The band gaps for Li4GeS4 and Li4SnS4 are 4.13eV and 3.54eV 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 Li4GeS4 and a direct band gap for Li4SnS4. The (Sn/Ge)–S bonds were determined to have significant covalent bonding character, while the Li–S bonds are primarily ionic in nature.