Applying band gap engineering to tune the linear optical and nonlinear optical properties of noncentrosymmetric chalcogenides $La_4Ge_3Se_X S_{12- x} (x = 0, 2, 4, 6, 8, 10)$

Abstract

$La_4Ge_3S_{12}$ was structurally characterized in 1974 without any optical or nonlinear optical properties reported. Herein we report that the acentric crystal structure of $La_4Ge_3S_{12}$ 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 $La_4Ge_3S_{12}$ are dominantly contributed by sulfur atoms. Band gap engineering was applied to tune the optical properties of $La_4Ge_3S_{12}$ by replacing S with Se, which resulted in five new compounds: $La_4Ge_3Se_2S_{10}$, $La_4Ge_3Se_4S_8$, $La_4Ge_3Se_6S_6$, $La_4Ge_3Se_8S_4$, and $La_4Ge_3Se_{10}S_2$. High quality mm-sized crystals of $La_4Ge_3Se_xS_{12−x} (x = 0, 2, 4, 6, 8, 10)$ were grown by a high-temperature vapor transport reaction followed by salt flux growth methods. All $La_4Ge_3Se_xS_{12−x} (x = 0, 2, 4, 6, 8, 10)$ compounds are isostructural and crystallize in the noncentrosymmetric rhombohedral space group R3c (no. 161) with unit cell volumes of $2639.2 Å^3, 2698.6 Å^3, 2745.3 Å^3, 2792.6 Å^3, 2845.3 Å^3$, and $2912.1 Å^3$, for x = 0, 2, 4, 6, 8, and 10, respectively. $La_4Ge_3S_{12}$ shows a band gap of 3.1(1) eV, and the incorporation of Se to the system suppresses the band gap of the compound $La_4Ge_3Se_{10}S_2$ to 1.9(1) eV. The incorporation of Se significantly shifts the nonlinear optical properties of $La_4Ge_3S_{12}$, with an example of the SHG response of $La_4Ge_3Se_4S_8$ being four times higher than $La_4Ge_3S_{12}$. The optical and nonlinear optical properties of $La_4Ge_3Se_xS_{12−x} (x = 0, 2, 4, 6, 8, 10)$ compounds are reported.