Layered Ternary Indium Chalcogenides Ba₅In₂S₈ and β-BaIn₂Se₄: Structural Diversity and Large Birefringence in β-BaIn₂Se₄

Two new ternary indium chalcogenides Ba5In2S8 and β-BaIn2Se4 have been synthesized by solid-state reactions. Ba5In2S8 crystallizes in space group I4/mcm of the tetragonal system (a = 8.4391(6) Å, c = 23.290(2) Å, V = 1658.6(3) Å3, Z = 4) featuring a two-dimensional layered structure consisting of corner-sharing In1-centered tetrahedra, isolated In2S4 units, and charge-compensating Ba2+ cations. β-BaIn2Se4 adopts the monoclinic symmetry with space group C2/m (a = 13.0883(8) Å, b = 22.5644(13) Å c = 6.5442(4) Å, β = 117.657 (2)°, V = 1711.87(18) Å3, Z = 8). Its structure was characterized by two-dimensional slabs in which corner- and edge-shared [InSe4] tetrahedra create an extended framework, with the charge-balancing Ba2+ ions located in the interlayer spaces. Diffuse reflectance spectroscopy yields optical band gaps of 2.60 eV for Ba5In2S8 and 2.52 eV for β-BaIn2Se4, demonstrating a slight narrowing upon selenium substitution. The band structure diagrams reveal that both compounds belong to direct bandgap semiconductors. The PDOS analyses reveal that the valence band maximum of Ba5In2S8 is primarily composed of Ba atomic orbitals, whereas the conduction band minimum arises mainly from S atomic orbitals. Conversely, the valence band maximum and the conduction band minimum of β- BaIn2Se4 are predominantly governed by Se and In atomic orbitals, respectively. The calculated birefringence values of Ba5In2S8 and β-BaIn2Se4 are 0.05 and 0.106 @1064 nm, respectively. The large birefringence of β-BaIn2Se4 suggests that it is a potential infrared birefringent material.