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

Optical anisotropy, as the core physical property for polarization manipulation, has always posed a significant challenge in the design of functional optical materials regarding its regulation mechanism and performance optimization. In the mid-far infrared (IR) region, optical materials that possess both large birefringence and wide transparent range are extremely scarce. In this study, we synthesized Hg ₉ Ga ₄ Se ₄ Cl ₁₆ (HGSC), a tridymite-like topological structure incorporating well-aligned linear [Hg ₃ Se ₂ ] units. HGSC exhibits an exceptional birefringence of 0.808 at 546 nm, which is 67 times higher than that of commercial MgF ₂ , while also possessing 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 [Hg ₃ Se ₂ ] clusters, which exhibit the highest optical anisotropy (𝛿 = 430) among all known birefringence-active functional units. This study presents a new bifunctional unit for the design of optical materials that combine both a wide IR transparency range and exceptional birefringence. Furthermore, as the first selenide to feature [Hg ₃ ] ²⁺ atomic multinuclear clusters, HGSC compounds offer significant potential for applications in thermoelectric, magnetism, and low-temperature superconductivity.