Flux Crystal Growth of Ruddlesden–Popper Titanium Oxysulfides Li Ln TiO 3 S ( Ln = La–Gd): Lithium-Ion Mobility in Li 1– x Nd(Ti 1– x Nb x )O 3 S ( x = 0–0.25)

High Resolution Image Download MS PowerPoint Slide We report the flux crystal growth of n = 1 Ruddlesden–Popper type titanium oxysulfides, Li Ln TiO 3 S ( Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), using alkali metal halide molten salts. The transparent platelet Li Ln TiO 3 S single crystals were pale yellow or orange, except for dark purple LiCeTiO 3 S. All Li Ln TiO 3 S members are stable in air, particularly up to 600 °C for Ln = Nd. Single crystal structure analysis revealed that Li Ln TiO 3 S adopted a centrosymmetric tetragonal cell in the space group P 4/ nmm (no. 129) for Ln = La to Gd, which is in contrast to the corresponding oxide Li Ln TiO 4, which exhibits a structural symmetry lowering to P -42 1 m for Ln = Sm–Dy via inversion symmetry breaking. The structure of Li Ln TiO 3 S consists of single perovskite layers composed of corner-sharing TiO 5 S octahedra, with (LiO) 2 antifluorite layers and ( Ln S) 2 rock-salt layers stacked above and below along the c -axis. The lithium-ion conductivity of sintered polycrystalline LiNdTiO 3 S was significantly enhanced by the substitution of Nb 5+ for Ti 4+, which induced lithium vacancies. When x = 0.20, the bulk ionic conductivity increased from 3.00 × 10 –7 S cm –1 at 25 °C to 1.46 × 10 –2 S cm –1 at 500 °C, and the activation energy was 0.495 eV, which is lower than those of any other Ruddlesden–Popper type oxide-based solid electrolytes. Anion-directed material design using both oxide and sulfide ions provides opportunities for creating distinct classes of lithium-ion solid electrolytes.