NbGa ₂ S ₄ (S ₂ ): A Rare Linear [S ₂ ] ²⁻ Coordination to Niobium Resulting in Simultaneously Large SHG Effect and Giant Birefringence

We report a novel niobium thiogallate, NbGa₂S₄(S₂), featuring a rare linear [S₂]^2- coordination to niobium, which simultaneously exhibits a strong second harmonic generation effect [1.5 × benchmark AgGaS₂ (AGS)], a high laser-induced damage threshold (5.7 × AGS), and a giant birefringence (0.29 at 2 µm, the largest among all reported 3D chalcogenides). Theoretical studies reveal that the polarizability anisotropy and hyperpolarizability are significantly enhanced due to the unique alignment of [S₂]^2- dimer within the crystal lattice. Additionally, the compound demonstrates exceptional chemical bond flexibility, which leads to significantly reduced thermal anisotropic expansion, one order of magnitude smaller than that of the benchmark AGS. These findings provide valuable insights for the design of next-generation functional materials with tailored optical and thermal characteristics.