Synthesis,\nOptical, and Magnetic Properties of the\nMixed Chalcogenide Semiconductor Series Eu(II)<sub>2</sub>SiSe<sub><i>x</i></sub>S<sub>4‑x</sub>, Prepared via the\nFlux-Assisted Boron Chalcogen Mix

We report a detailed structural study of a series of\nfive new quaternary\nEu(II)-containing mixed chalcogenide phases, Eu<sub>2</sub>SiSe<sub>0.85</sub>S<sub>3.15</sub>, Eu<sub>2</sub>SiSe<sub>2.4</sub>S<sub>1.6</sub>, Eu<sub>2</sub>SiSe<sub>2.6</sub>S<sub>1.4</sub>, Eu<sub>2</sub>SiSe<sub>3.1</sub>S<sub>0.9</sub>, and Eu<sub>2</sub>SiSe<sub>4</sub>, synthesized using the flux-assisted boron chalcogen mixture\n(BCM) method. High-quality crystals were grown, and their crystal\nstructures were determined by single-crystal X-ray diffraction. All\nmembers of the Eu<sub>2</sub>SiSe<sub><i>x</i></sub>S<sub>4−<i>x</i></sub> series crystallize in the monoclinic\ncrystal system with space group <i>P</i>2<sub>1</sub>/<i>m</i>, except Eu<sub>2</sub>SiSe<sub>4</sub>, which crystallizes\nin the <i>P</i>2<sub>1</sub> space group. The crystal structure\nof Eu<sub>2</sub>SiSe<sub><i>x</i></sub>S<sub>4−<i>x</i></sub> exhibits a complex three-dimensional network and\nis primarily composed of distorted trigonal prismatic Eu<i>Q</i><sub>6</sub> polyhedra that are interconnected by Si<i>Q</i><sub>4</sub> tetrahedra. Polycrystalline powders were used for physical\nproperty measurements, including the magnetic susceptibility and UV–vis\ndiffuse reflectance. Magnetic measurements indicated paramagnetic\nbehavior with a slightly negative Weiss constant (θ = −13.54).\nThe band gaps of the materials were determined from diffuse reflectance\ndata, with optical band gaps estimated to be 2.04(2) eV, 1.98(2) eV,\nand 1.90(2) eV for Eu<sub>2</sub>SiSe<sub>0.85</sub>S<sub>3.15</sub>, Eu<sub>2</sub>SiSe<sub>2.6</sub>S<sub>1.4</sub>, and Eu<sub>2</sub>SiSe<sub>4</sub>, respectively. Band gap tuning was achieved by partially\nor fully replacing the S sites with Se.