Mid- and far-infrared birefringent crystals are key functional materials for polarization control, laser technologies, and infrared photonics. However, existing materials generally suffer from limited infrared transparency, an intrinsic trade-off between large birefringence and wide transmission windows, and challenges in optical characterization due to restricted crystal dimensions.

Owing to their heavy-element composition and unique coordination environments, Hg-based chalcogenides have emerged as promising candidates for achieving the coexistence of strong birefringence and broad infrared transparency. Nevertheless, a systematic understanding of their structural design principles and structure-property relationships remains lacking.

To address this knowledge gap, a research team from the Xinjiang Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences has developed a novel Hg-based chalcogenide—Hg₁₈Ga₈Se₈Cl₃₂ (HGSC)—featuring linear [Hg₃Se₂] building units. The team systematically investigated the crystal's structure, optical properties, and thermal response behavior.

By combining experimental characterizations with theoretical calculations, the researchers elucidated the structural origin underlying the synergistic realization of giant birefringence and ultrabroad infrared transparency. They also proposed an infrared birefringence measurement strategy suitable for small-sized crystals, offering new insights for the design and characterization of next-generation mid- and far-infrared birefringent materials.

Notably, the team for the first time constructed linear multinuclear [Hg₃Se₂] cluster building units in an Hg-based chalcogenide system. These linear clusters are highly ordered along a specific crystallographic direction, forming a pseudo-tridymite-like topological framework. Unlike conventional tetrahedral or chain-like chalcogenide structures, the linear Hg clusters exhibit pronounced orientational consistency within the crystal lattice, providing a new structural design paradigm for achieving strong optical anisotropy.

The synthesized HGSC single crystals simultaneously exhibit an ultrabroad infrared transparent window spanning 0.4–25 μm (covering key mid- and far-infrared atmospheric windows) and a giant birefringence of up to 0.871 at 546.1 nm. Importantly, the crystal maintains a high birefringence value of 0.453 at 3.5 μm. This combination of optical properties ranks among the best reported for Hg-based chalcogenides and infrared birefringent crystals, effectively overcoming the long-standing challenge of concurrently achieving large birefringence and wide infrared transparency.

Furthermore, given that newly developed infrared crystals are often only available in millimeter-scale sizes—rendering conventional mid-infrared birefringence measurement techniques impractical—the work proposes and validates an infrared birefringence measurement method based on polarization-state modulation and phase-retardation analysis. By precisely analyzing the phase difference between two orthogonal polarization components, this approach enables quantitative determination of birefringence in small-sized crystals across the near-infrared to mid-infrared spectral regions.

Through the integration of variable-temperature single-crystal X-ray diffraction, in situ Raman spectroscopy, and first-principles calculations, the team systematically revealed the microscopic origin of HGSC's outstanding optical properties. Both experimental observations and theoretical analyses confirm that the linear [Hg₃Se₂] units possess the highest polarizability anisotropy (δ ≈ 430) among all known birefringence-active structural units. Their highly oriented arrangement within the crystal lattice significantly amplifies the material's overall birefringence. Meanwhile, a dynamic lattice distortion mechanism dominated by electron-phonon coupling accounts for the material's reversible thermochromic behavior.

The research was recently published in Nature Communications. It was supported by the National Key Research and Development Program of China, the Shanghai Cooperation Organization Science and Technology Partnership Program, and other funding sources.