Flux Crystal Growth, Structure, and Optical Properties of LiLa₃Ti₂S₃O₆: An Oxysulfide Phase Derived from K₂NiF₄-Type...

Single crystals of a new titanium oxysulfide, LiLa₃Ti₂S₃O₆, were grown from a KI molten salt. Single-crystal X-ray diffraction analysis revealed that LiLa₃Ti₂S₃O₆ crystallizes in the space group Pnma with lattice parameters of a = 11.7319(4) Å, b = 3.94787(14) Å, and c = 20.6885(6) Å. In this structure, the one-dimensional chains of corner-sharing TiO₅S octahedra are further corner-linked via equatorial and apical oxygen atoms to form unique corrugated two-dimensional perovskite-type layers in the ab plane with one octahedral thickness. These layers were intervened along the c-axis by the LaS rock-salt layers corrugated concomitantly with the perovskite-type layers, and LiO₂S₂ tetrahedral chains were located between these two types of two-dimensional layers. LiLa₃Ti₂S₃O₆ can be viewed as a modified K₂NiF₄-type structure with TiO₅S octahedral layers stacked in a zigzag manner along the c axis. The oxysulfide has a direct-type band gap of 1.85 eV, based on UV-vis-NIR diffuse reflectance measurements. First-principles calculations showed that the conduction band minimum mainly consists of Ti 3d orbitals, and the valence band maximum consists of S 3p, O 2p, and Li 2s orbitals. The electronic structures near the Fermi level are similar to those of the structurally related photocatalytic oxysulfides Y₂Ti₂S₂O₅ and La₅Ti₂CuS₅O₇.