LiGa₂PS₆ and LiCd₃PS₆: Molecular Designs of Two New Mid-Infrared Nonlinear Optical Materials

To simultaneously maintain large second harmonic generation (SHG) and high laser damage thresholds (LDTs) for mid- and far-infrared (IR) nonlinear optical (NLO) materials is very challenging and significant in laser science and technology. Using a chemical substitution strategy, two new mid-infrared NLO materials based on metal thiophosphates, namely, LiGa2PS6 and LiCd3PS6, have been obtained from AgGa2PS6. They both crystallize in the Cc space group but exhibit different structures. LiGa2PS6 is isostructural with the parent AgGa2PS6, displaying a three-dimensional (3D) anionic framework of [Ga2PS6]− composed of [Ga2S6]6– chains and PS4 tetrahedra with the 1D triangular-shaped tunnels filled by Li+ ions. LiCd3PS6 features a 3D anionic framework of [Cd3S6]6– composed of 2D layers of [Cd2S5]6– and the bridging CdS4 units with 1D tunnels along the b-axis filled by [LiPS6]6– chains. Remarkably, both materials display large band gaps of 3.15 eV (for LiGa2PS6) and 2.97 eV (for LiCd3PS6), and keep a good balance between strong SHG responses (0.5–0.8 × AgGaS2 at 1910 nm in the particle range 53–71 μm) and high LDTs (5.5–10.4 × AgGaS2); therefore both crystals are new promising mid-IR NLO materials.