PbGa₂GeS₆: An Infrared Nonlinear Optical Material Synthesized by an Intermediate-Temperature Self-Fluxing Method

A new non-centrosymmetric (NCS) polar sulfide PbGa₂GeS₆ has been synthesized by a self-fluxing method at a relatively low temperature of 550 °C with desirable properties for nonlinear optical (NLO) applications. Its structure is built by three types of infinite chains intersecting in three dimensions, where the NCS building units are tetragonal pyramids of PbS₄ and tetrahedra of MS₄ (M = Ga, Ga/Ge). Powder of PbGa₂GeS₆ exhibits phase-matchable (PM) second-order NLO activity with strong second harmonic generation (SHG) intensity of 0.5× AgGaS₂ at the particle size of 150–210 μm, high laser-induced damage threshold (LIDT) of 5× AgGaS₂, and a wide transmission range in infrared (IR) region (0.47–23 μm). First-principles calculations suggest that the macroscopic SHG response is originated from the cooperation of the lone pairs on Pb²⁺ and MS₄ (M = Ga, Ga/Ge) tetrahedra. Considering its strong PM SHG response, high LIDT, wide IR transmission range, and relatively low synthesis temperature, PbGa₂GeS₆ should be a promising candidate for high-power IR NLO applications