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)

Abstract

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 La₄Ge₃Se_xS_12−x (x = 0, 2, 4, 6, 8, 10) were grown by a high-temperature vapor transport reaction followed by salt flux growth methods. All La₄Ge₃Se_xS_12−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 ų, 2698.6 ų, 2745.3 ų, 2792.6 ų, 2845.3 ų, and 2912.1 ų, 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 La₄Ge₃Se_xS_12−x (x = 0, 2, 4, 6, 8, 10) compounds are reported.