Applying band gap engineering to tune the linear optical and nonlinear optical properties of noncentrosymmetric chalcogenides La4Ge3SexS12-x (x=0, 2, 4, 6, 8, 10)

La4Ge3S12 was structurally characterized in 1974 without any optical or nonlinear optical properties reported. Herein we report that the acentric crystal structure of La4Ge3S12 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 La4Ge3S12 are dominantly contributed by sulfur atoms. Band gap engineering was applied to tune the optical properties of La4Ge3S12 by replacing S with Se, which resulted in five new compounds: La4Ge3Se2S10, La4Ge3Se4S8, La4Ge3Se6S6, La4Ge3Se8S4, and La4Ge3Se10S2. High quality mm-sized crystals of La4Ge3SexS12-x (x = 0, 2, 4, 6, 8, 10) were grown by a high-temperature vapor transport reaction followed by salt flux growth methods. All La4Ge3SexS12-x (x = 0, 2, 4, 6, 8, 10) compounds are isostructural and crystallize in the noncentrosymmetric rhombohedral space group R3c (no. 161) with unit cell volumes of 2639.2 angstrom(3), 2698.6 angstrom(3), 2745.3 angstrom(3), 2792.6 angstrom(3), 2845.3 angstrom(3), and 2912.1 angstrom(3), for x = 0, 2, 4, 6, 8, and 10, respectively. La4Ge3S12 shows a band gap of 3.1(1) eV, and the incorporation of Se to the system suppresses the band gap of the compound La4Ge3Se10S2 to 1.9(1) eV. The incorporation of Se significantly shifts the nonlinear optical properties of La4Ge3S12, with an example of the SHG response of La4Ge3Se4S8 being four times higher than La4Ge3S12. The optical and nonlinear optical properties of La4Ge3SexS12-x (x = 0, 2, 4, 6, 8, 10) compounds are reported.