Crystal Growth of Quaternary AkRE2Si2S8 (Ak = Ca and Sr; RE = La–Tb) Thiosilicates Using Flux-Assisted Boron Chalcogen Mixture Method: Exploring X-ray Scintillation, Luminescence, and Magnetic Properties

We report on the detailed structural analysis of a series of 11 new quaternary rare earths containing thiosilicates, AkRE₂Si₂S₈ (Ak = Ca and Sr; RE = La, Ce, Pr, Nd, Sm, Gd, and Tb), synthesized using the flux-assisted boron chalcogen mixture method. High quality crystals were grown and used to determine their crystal structures by single crystal X-ray diffraction. All members of the AkRE₂Si₂S₈ series crystallize in the trigonal crystal system with space group R3̅c (space group no. 167). Polycrystalline powders were used for physical property measurements, including magnetic susceptibility, diffuse reflectance in the UV–visible range, and scintillation. Magnetic measurements indicated that CaRE₂Si₂S₈ (RE = Nd and Tb) exhibits paramagnetic behavior with a slightly negative Weiss constant. The band gaps of the materials were determined from diffuse reflectance data, and optical band gaps were estimated to be 2.5(1) and 2.9(1) eV for CaCe₂Si₂S₈ and CaGd₂Si₂S₈, respectively. CaCe₂Si₂S₈, CaTb₂Si₂S₈, and SrCe₂Si₂S₈ exhibited intense green luminescence upon irradiation with 375 nm ultraviolet light and, furthermore, scintillated when exposed to X-rays. Radioluminescence measurements of CaCe₂Si₂S₈ powder revealed green emission with an intensity approximately 14% of that emitted by bismuth germanium oxide powder.