Synthesis, Optical, and Magnetic Properties of the Mixed Chalcogenide Semiconductor Series Eu(II)₂SiSe_<i>x</i>S_4-x, Prepared via the Flux-Assisted Boron Chalcogen Mixture Method

We report a detailed structural study of a series of five new quaternary Eu(II)-containing mixed chalcogenide phases, Eu₂SiSe_0.85S_3.15, Eu₂SiSe_2.4S_1.6, Eu₂SiSe_2.6S_1.4, Eu₂SiSe_3.1S_0.9, and Eu₂SiSe₄, synthesized using the flux-assisted boron chalcogen mixture (BCM) method. High-quality crystals were grown, and their crystal structures were determined by single-crystal X-ray diffraction. All members of the Eu₂SiSe_<i>x</i>S_4-<i>x</i> series crystallize in the monoclinic crystal system with space group <i>P</i>2₁/<i>m</i>, except Eu₂SiSe₄, which crystallizes in the <i>P</i>2₁ space group. The crystal structure of Eu₂SiSe_<i>x</i>S_4-<i>x</i> exhibits a complex three-dimensional network and is primarily composed of distorted trigonal prismatic Eu<i>Q</i>₆ polyhedra that are interconnected by Si<i>Q</i>₄ tetrahedra. Polycrystalline powders were used for physical property measurements, including the magnetic susceptibility and UV-vis diffuse reflectance. Magnetic measurements indicated paramagnetic behavior with a slightly negative Weiss constant (θ = -13.54). The band gaps of the materials were determined from diffuse reflectance data, with optical band gaps estimated to be 2.04(2) eV, 1.98(2) eV, and 1.90(2) eV for Eu₂SiSe_0.85S_3.15, Eu₂SiSe_2.6S_1.4, and Eu₂SiSe₄, respectively. Band gap tuning was achieved by partially or fully replacing the S sites with Se.