KHC₂O₄·B(OH)₃: A nonlinear optical crystal with large birefringence in the short‐wavelength ultraviolet region

Exploring nonlinear optical crystals with large birefringence in the short‐wavelength ultraviolet (SWUV) regions (≤300 nm) is a valuable pursuit. Herein, KHC 2 O 4 ·B(OH) 3 , which is composed of infinite [HC 2 O 4 ] − and [B(OH) 3 ] chains, was successfully grown via the slow evaporation method. The [HC 2 O 4 ] − and [B(OH) 3 ] chains endow KHC 2 O 4 ·B(OH) 3 with the largest birefringence (0.164@1064 nm) of all B(OH) 3 ‐containing compounds that have been reported in the SWUV regions. Moreover, it possesses a moderate second‐harmonic generation response (0.3 × KH 2 PO 4 ) and a short UV cutoff edge of 254 nm. The electron localization function diagram indicates the large birefringence originates from the HC 2 O 4 and B(OH) 3 groups. This work provides a new method to investigate crystals with large birefringence in the B(OH) 3 ‐based oxalate system.