Flux Crystal Growth of Ruddlesden–Popper Titanium Oxysulfides Li <i>Ln</i> TiO ₃ S ( <i>Ln</i> = La–Gd): Lithium-Ion Mobility in Li _{1– <i>x</i>} Nd(Ti _{1– <i>x</i>} Nb _<i>x</i> )O ₃ S ( <i>x</

We report the flux crystal growth of n = 1 Ruddlesden–Popper type titanium oxysulfides, LiLnTiO3S (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), using alkali metal halide molten salts. The transparent platelet LiLnTiO3S single crystals were pale yellow or orange, except for dark purple LiCeTiO3S. All LiLnTiO3S members are stable in air, particularly up to 600 °C for Ln = Nd. Single crystal structure analysis revealed that LiLnTiO3S adopted a centrosymmetric tetragonal cell in the space group P4/nmm (no. 129) for Ln = La to Gd, which is in contrast to the corresponding oxide LiLnTiO4, which exhibits a structural symmetry lowering to P-421m for Ln = Sm–Dy via inversion symmetry breaking. The structure of LiLnTiO3S consists of single perovskite layers composed of corner-sharing TiO5S octahedra, with (LiO)2 antifluorite layers and (LnS)2 rock-salt layers stacked above and below along the c-axis. The lithium-ion conductivity of sintered polycrystalline LiNdTiO3S was significantly enhanced by the substitution of Nb5+ for Ti4+, 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.