Crystal Growth of Quaternary AkRE 2 Si 2 S 8 (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, AkRE 2 Si 2 S 8 (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 AkRE 2 Si 2 S 8 series crystallize in the trigonal crystal system with space group R 3̅ c (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 CaRE 2 Si 2 S 8 (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 CaCe 2 Si 2 S 8 and CaGd 2 Si 2 S 8 , respectively. CaCe 2 Si 2 S 8 , CaTb 2 Si 2 S 8 , and SrCe 2 Si 2 S 8 exhibited intense green luminescence upon irradiation with 375 nm ultraviolet light and, furthermore, scintillated when exposed to X-rays. Radioluminescence measurements of CaCe 2 Si 2 S 8 powder revealed green emission with an intensity approximately 14% of that emitted by bismuth germanium oxide powder.