Syntheses, Structures, and Nonlinear-Optical Properties of Metal Sulfides Ba₂Ga₈MS₁₆ (M = Si, Ge)

Two new metal sulfides, Ba2Ga8MS16 (M = Si, Ge), have been synthesized by high-temperature solid-state reactions. They are isostructural and crystallize in the noncentrosymmetric space group P63mc (No. 186) with a = 10.866(5) Å, c = 11.919(8) Å, and z = 2 for Ba2Ga8SiS16 (1) and a = 10.886(8) Å, c = 11.915(3) Å, and z = 2 for Ba2Ga8GeS16 (2). Their three-dimensional frameworks are constructed by corner-sharing mixed (Ga/M)­S4 (M = Si, Ge) and pure GaS4 tetrahedra, with Ba2+ cations filling in the tunnels. Compounds 1 and 2 are transparent over 0.42–20 μm and have wide band gaps of around 3.4 and 3.0 eV, respectively. Polycrystalline 2 displays strong nonlinear second-harmonic-generation (SHG) intensities that are comparable to that of the benchmark AgGaS2 (AGS) with phase-matching behavior at a laser irradiation of 1950 nm. Of particular interest, compound 2 also possesses a high powder laser-induced damage threshold of ∼22 times that of AGS. The alternate stacking of the mixed (Ga/M)­S4 (M = Si, Ge) tetrahedral layer with the pure GaS4 tetrahedral layer along the c axis and the alignment of these two types of tetrahedra in the same direction may be responsible for the large SHG signals observed.