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) ų, 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) ų, 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.