[Hg₃Se₂]^2- cluster drives giant optical anisotropy and broad infrared transparency

Optical anisotropy, a fundamental physical property for polarization control, has long presented a critical consideration in the development of optical materials, particularly in terms of its modulation mechanisms and performance optimization. In the mid- to far-infrared region, simultaneously achieving large birefringence and broad transparency within a single material remains a major challenge. Herein, we report the synthesis of Hg₁₈Ga₈Se₈Cl₃₂ (HGSC), a crystalline material featuring linear [Hg₃Se₂] structural units. HGSC demonstrates a large birefringence of 0.871 at 546 nm, accompanied by the broadest transparency window among Hg-based chalcogenide single crystals (0.4 to 25 µm). Theoretical calculations reveal that the significant birefringence of HGSC originates from the well-aligned linear [Hg₃Se₂]^2- clusters, which exhibit the highest polarizability anisotropy (δ = 430) among all reported birefringence-active functional units. The demonstration of [Hg₃Se₂]^2- clusters as an effective bifunctional unit offers new opportunities for designing infrared photonic materials.