Density-Compensated Polarizability Anisotropy for Record Birefringence in Short-Wavelength Ultraviolet Inorganic Crystals.

Birefringent crystals are indispensable for polarization manipulation and phase-matching in advanced photonic and optical technologies. However, the development of short-wavelength ultraviolet (SWUV) birefringent materials remains challenging, attributed to the intrinsic conflict between wide bandgaps and strong optical anisotropy. Herein, we propose a density-compensated polarizability anisotropy strategy to overcome this limitation. The linear cyanate anion, featuring a large HOMO-LUMO gap and a small van der Waals volume, is identified as an ideal functional unit and combined with d10 transition-metal cationic anchors to construct four new inorganic birefringent crystals with SWUV transparency: K[Hg(NCO)2]Cl (1), K[Cd(NCO)3] (2), K2[Hg(NCO)4] (3), and K2[Cd(NCO)4] (4). Compounds 1-4 exhibit SWUV cutoff edges at 254 (1), 228 (2), 253 (3), and 229 nm (4), together with large birefringence values of 0.273 (1), 0.329 (2), 0.524 (3), and 0.511 (4) at 546 nm. The enhanced birefringence originates from the increased density and optimized alignment of linear (NCO)- groups within the crystal lattices. Notably, the small van der Waals volume of (NCO)- enables exceptionally high group densities and well-aligned arrangements in compounds 3 and 4, endowing them with record-high birefringence among all reported SWUV inorganic birefringent crystals. This work demonstrates the strong potential of linear cyanate groups for SWUV birefringent applications and establishes a general density-compensated polarizability anisotropy strategy based on linear functional units, providing a new paradigm for the rational design of high-performance inorganic birefringent crystals that simultaneously achieve large birefringence and wide bandgaps.