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 d 10 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 ), K 2 [Hg(NCO) 4 ] ( 3 ), and K 2 [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.