Pb3.5GeS4Br3: the first phase-matching thiogermanate halide infrared nonlinear optical material

Abstract

Two new thiohalides Pb₃GeS₄Br₂ and Pb_3.5GeS₄Br₃ have been synthesized by introducing Pb²⁺ into the thiogermanate system. The compounds crystallize in centrosymmetric P2₁/c (Pb₃GeS₄Br₂) and non-centrosymmetric P6₃ (Pb_3.5GeS₄Br₃) space groups, respectively. To the best of our knowledge, Pb₃GeS₄Br₂ and Pb_3.5GeS₄Br₃ are the first Pb-containing thiogermanate halides, and the latter is the first phase-matching IR nonlinear optical material in the thiogermanate halide system. Due to the presence of Pb²⁺ with stereochemically active lone pair electrons, Pb₃GeS₄Br₂ exhibits a strong optical anisotropy with a birefringence of 0.131@1064 nm, while Pb_3.5GeS₄Br₃ shows a large second-harmonic generation response (0.8 × AgGaS₂) and high laser-induced damage threshold (3.0 × AgGaS₂). These results enrich the structural and chemical diversity of chalcohalides.