was structurally characterized in 1974 without any optical or nonlinear optical properties reported. Herein we report that the acentric crystal structure of has been confirmed by single-crystal X-ray diffraction. The nonlinear optical properties test revealed its phase-matchable behavior with a good balance between the second harmonic generation (SHG) response and laser damage threshold (LDT). Theory calculations predicted that the optical properties of are dominantly contributed by sulfur atoms. Band gap engineering was applied to tune the optical properties of by replacing S with Se, which resulted in five new compounds: , , , , and . High quality mm-sized crystals of were grown by a high-temperature vapor transport reaction followed by salt flux growth methods. All compounds are isostructural and crystallize in the noncentrosymmetric rhombohedral space group R3c (no. 161) with unit cell volumes of , and , for x = 0, 2, 4, 6, 8, and 10, respectively. shows a band gap of 3.1(1) eV, and the incorporation of Se to the system suppresses the band gap of the compound to 1.9(1) eV. The incorporation of Se significantly shifts the nonlinear optical properties of , with an example of the SHG response of being four times higher than . The optical and nonlinear optical properties of compounds are reported.