Flux-Assisted Boron Chalcogen Mixture (BCM) Method for Synthesizing Mixed Chalcogenide Semiconductors (AkRE ₂ Si ₂ Se _<i>x</i> S _{8– <i>x</i>} and CaRE ₂ Si ₂ Se ₈ ) ( <i>

We report a detailed structural analysis of a series of ten quaternary rare earth-containing seleno-thiosilicates <i>AkRE</i>₂Si₂Se_<i>x</i>S_8-<i>x</i> and selenosilicates, Ca<i>RE</i>₂Si₂Se₈ (<i>Ak</i> = Ca and Sr; <i>RE</i> = La, Ce, Pr, Nd, and Sm). Single crystals were obtained by using the flux-assisted boron chalcogen mixture (BCM) method, and single-crystal X-ray diffraction was used to determine their structures. All members of the <i>AkRE</i>₂Si₂Se_<i>x</i>S_8-<i>x</i> and Ca<i>RE</i>₂Si₂Se₈ series crystallize in the space group <i>R</i>3̅<i>c</i> (space group number 167) of the trigonal crystal system. The single-crystal X-ray diffraction analysis revealed a strong preference for Se/S atoms to occupy one vs the other of the two available sites. Polycrystalline samples were used for the magnetic susceptibility and UV-visible diffuse reflectance measurements. Magnetic measurements show that CaCe₂Si₂Se_1.73S_6.27 and CaNd₂Si₂Se_2.5S_5.5 are paramagnetic with negative Weiss constants (θ = -60.1 and -26.2). Diffuse reflectance analysis gives optical band gaps of 2.7(1) eV (CaLa₂Si₂Se_2.38S_5.62), 2.2(1) eV (CaCe₂Si₂Se_1.73S_6.27), 2.5(1) eV (CaNd₂Si₂Se₂.₅S_5.5), and 2.0(1) eV (CaCe₂Si₂Se₈), consistent with density functional theory calculations. By partially or fully replacing S sites with Se, it was possible to achieve band gap tuning. Photoluminescence behavior was also investigated for CaCe₂Si₂Se_1.73S_6.27 via irradiation with 375 nm ultraviolet light.