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

Li4SnS4 and Li4GeS4 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 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 degrees C (Li4GeS4) and 958 degrees C (Li4SnS4). The band gaps for Li4GeS4 and Li4SnS4 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 Li4GeS4 and a direct band gap for Li4SnS4. The (Sn/Ge)-S bonds were determined to have significant covalent bonding character, while the LiS bonds are primarily ionic in nature. (C) 2013 Elsevier B.V. All rights reserved.