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 <i>density-compensated polarizability anisotropy</i> 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 <i>d</i>¹⁰ transition-metal cationic anchors to construct four new inorganic birefringent crystals with SWUV transparency: K[Hg(NCO)₂]Cl (<b>1</b>), K[Cd(NCO)₃] (<b>2</b>), K₂[Hg(NCO)₄] (<b>3</b>), and K₂[Cd(NCO)₄] (<b>4</b>). Compounds <b>1</b>-<b>4</b> exhibit SWUV cutoff edges at 254 (<b>1</b>), 228 (<b>2</b>), 253 (<b>3</b>), and 229 nm (<b>4</b>), together with large birefringence values of 0.273 (<b>1</b>), 0.329 (<b>2</b>), 0.524 (<b>3</b>), and 0.511 (<b>4</b>) 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 <b>3</b> and <b>4</b>, 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 <i>density-compensated polarizability anisotropy</i> 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.