[Ba₄X][In₁₉S₃₂] (X = Cl, Br): two quaternary metal chalcohalides exhibiting remarkable photocurrent responses

Inorganic metal chalcohalides, as significant semiconductor materials, have emerged as promising candidates for photoelectric applications. Herein, a new type of quaternary chalcohalide, [Ba₄X][In₁₉S₃₂] (X = Cl, Br), has been discovered using the high-temperature halide salt flux method. Single-crystal X-ray diffraction analysis reveals that they are isostructural and crystallize in the tetragonal space group I4₁/amd (no. 141) featuring the octahedral hole formed by six [InS₄]^5- tetrahedra filled with a [ClBa₄]⁷⁺ polycation, surrounded by a three-dimensional covalent framework formed by interconnecting [InS₆]^9- octahedra through corner-sharing and edge-sharing. Moreover, [Ba₄Cl][In₁₉S₃₂] and [Ba₄Br][In₁₉S₃₂] exhibit wide optical bandgaps of 2.70 eV and 2.46 eV, respectively, and moderate birefringences (0.044 @ 2100 nm and 0.042 @ 2100 nm, respectively). Specifically, [Ba₄X][In₁₉S₃₂] (X = Cl, Br) display remarkable photocurrent responses under simulated solar-light illumination, implying their potential for photocatalytic applications. Theoretical calculations were employed to understand the interrelationship between the optical properties and electronic structure. The study on the synthesis and structure-property relationship analysis of inorganic metal chalcohalides provides new insight into the exploration of promising photoelectric materials.