Difluoro(oxalato)borates as Short-Wavelength Optical Crystals with Bifunctional [BF₂C₂O₄] Units

Discovery of crystals functionalized by optically active units is of great significance to laser science and industry since these species have the potential for frequency conversion and light modulation. This spurred researchers on to find new optical crystals with such functionality, especially in new unexplored systems. Herein, inspired by the idea of introducing different optically active units into one structure, difluoro­(oxalato)­borate was defined as a new source of short-wavelength ultraviolet (UV) optical crystals. Based on this, three difluoro­(oxalato)­borates with [BF₂C₂O₄] bifunctional units were synthesized and characterized. Among them, NaBF₂C₂O₄ shows a large nonlinear optical effect (1.1 × KDP), a wide band gap (E_g = 4.48 eV), and a large birefringence (Δn_exp. = 0.162@546.1 nm), making it a potential short-wavelength UV nonlinear optical crystal. Among them, LiBF₂C₂O₄ and Sr­(BF₂C₂O₄)­(BF₄), with giant birefringence (Δn_cal. = 0.317 and 0.130@546 nm) as well as wide band gaps (5.54 and 4.76 eV), were characterized as short-wavelength UV birefringent crystals. Theoretical analysis clarifies that [BF₂C₂O₄] constructed by both π- and non-π-conjugated units shows balanced microscopic properties and that it can be regarded as bifunctional units to cause a considerable nonlinear optical effect and birefringence simultaneously. Our study confirms the feasibility of exploring optical crystals with balanced performance in the difluoro­(oxalato)­borate system.