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 <i>P</i>6₃<i>mc</i> (No. 186) with <i>a</i> = 10.866(5) Å, <i>c</i> = 11.919(8) Å, and <i>z</i> = 2 for Ba₂Ga₈SiS₁₆ (<b>1</b>) and <i>a</i> = 10.886(8) Å, <i>c</i> = 11.915(3) Å, and <i>z</i> = 2 for Ba₂Ga₈GeS₁₆ (<b>2</b>). 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 <b>1</b> and <b>2</b> are transparent over 0.42–20 μm and have wide band gaps of around 3.4 and 3.0 eV, respectively. Polycrystalline <b>2</b> 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 <b>2</b> 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 <i>c</i> axis and the alignment of these two types of tetrahedra in the same direction may be responsible for the large SHG signals observed